Initial commit: Alpine 5G Router with Fibocom FM350-GL modem configuration
- Complete working configuration for FM350-GL modem - CYTA Cyprus APN setup (internet) - AT command reference and troubleshooting guide - Configuration scripts and documentation - RNDIS mode working with manual IP configuration
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5G_MODEM_TROUBLESHOOTING.md
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5G_MODEM_TROUBLESHOOTING.md
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# Fibocom FM350-GL 5G Modem Configuration Guide
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Complete reference for configuring the Fibocom FM350-GL modem on Alpine Linux.
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## Working Configuration
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| Setting | Value |
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|---------|-------|
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| **Modem** | Fibocom FM350-GL |
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| **USB ID** | 0e8d:7126 (Mode 40) |
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| **AT Port** | /dev/ttyUSB1 |
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| **Network Interface** | eth1 (RNDIS) |
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| **APN** | internet (CYTA Cyprus) |
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## AT Command Reference
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### Sending AT Commands
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The FM350-GL doesn't need `stty` configuration. Send commands directly:
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```bash
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# Pattern for sending AT commands
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cat /dev/ttyUSB1 & CAT_PID=$!
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sleep 0.3
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echo -e 'YOUR_COMMAND\r' > /dev/ttyUSB1
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sleep 2
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kill $CAT_PID 2>/dev/null
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```
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### Essential Commands
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| Command | Purpose | Example Response |
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|---------|---------|------------------|
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| `AT` | Test communication | `OK` |
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| `AT+CGMI` | Manufacturer | `Fibocom Wireless Inc.` |
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| `AT+CGMM` | Model | `FM350-GL` |
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| `AT+CSQ` | Signal strength | `+CSQ: 7, 99` (7 = moderate) |
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| `AT+CREG?` | Network registration | `+CREG: 0,1` (1 = registered) |
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| `AT+CEREG?` | LTE/5G registration | `+CEREG: 0,1` |
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| `AT+CCID` | SIM card ID | `+CCID: 893570...` |
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### APN Configuration
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```bash
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# Set APN (CYTA Cyprus uses "internet")
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echo -e 'AT+CGDCONT=1,"IP","internet"\r' > /dev/ttyUSB1
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# Verify APN is set
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echo -e 'AT+CGDCONT?\r' > /dev/ttyUSB1
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# Response: +CGDCONT: 1,"IP","internet",""...
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```
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### Connection Management
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```bash
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# Activate PDP context (start data connection)
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echo -e 'AT+CGACT=1,1\r' > /dev/ttyUSB1
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# Response: +CGEV: ME PDN ACT 1, OK
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# Get assigned IP address
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echo -e 'AT+CGPADDR=1\r' > /dev/ttyUSB1
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# Response: +CGPADDR: 1,"10.156.167.104",""
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# Get full connection details (DNS, MTU)
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echo -e 'AT+CGCONTRDP=1\r' > /dev/ttyUSB1
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# Response includes DNS servers
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```
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### USB Mode Control
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```bash
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# Check current USB mode
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echo -e 'AT+GTUSBMODE?\r' > /dev/ttyUSB1
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# Response: +GTUSBMODE: 40
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# Available modes
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echo -e 'AT+GTUSBMODE=?\r' > /dev/ttyUSB1
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# Response: +GTUSBMODE: (40,41)
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# Switch modes (requires modem reset)
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echo -e 'AT+GTUSBMODE=40\r' > /dev/ttyUSB1
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# Reset modem
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echo -e 'AT+CFUN=1,1\r' > /dev/ttyUSB1
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```
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## USB Modes
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| Mode | USB Product ID | Description | AT Commands |
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|------|----------------|-------------|-------------|
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| 40 | 0e8d:7126 | RNDIS mode | ✅ Work on ttyUSB1 |
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| 41 | 0e8d:7127 | Extended mode | ❌ Don't work |
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**Important:** Stay in Mode 40 for reliable AT command access.
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## Network Interface Configuration
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### Why DHCP Doesn't Work
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The RNDIS interface (`eth1`) does **not** provide DHCP. The modem manages the connection internally. You must:
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1. Get the IP from the modem via `AT+CGPADDR=1`
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2. Configure the interface manually
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### Manual Configuration
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```bash
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# Get IP from modem (replace with actual value)
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MODEM_IP="10.156.167.104"
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# Configure interface
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ip link set eth1 up
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ip addr flush dev eth1
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ip addr add $MODEM_IP/32 dev eth1
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ip route add default dev eth1 metric 50
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# Test
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ping -c 3 8.8.8.8
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```
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## CYTA Cyprus Settings
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| Setting | Value |
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|---------|-------|
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| APN | `internet` |
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| DNS Primary | 195.14.130.220 |
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| DNS Secondary | 195.14.154.100 |
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| Network Name | Vodafone |
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## Complete Connection Sequence
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```bash
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#!/bin/sh
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# Full connection sequence for FM350-GL with CYTA SIM
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AT_PORT="/dev/ttyUSB1"
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APN="internet"
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# 1. Configure APN
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cat $AT_PORT & PID=$!; sleep 0.3
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echo -e "AT+CGDCONT=1,\"IP\",\"$APN\"\r" > $AT_PORT
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sleep 2; kill $PID 2>/dev/null
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# 2. Activate connection
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cat $AT_PORT & PID=$!; sleep 0.3
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echo -e "AT+CGACT=1,1\r" > $AT_PORT
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sleep 3; kill $PID 2>/dev/null
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# 3. Get IP address
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MODEM_IP=$(timeout 5 sh -c "
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cat $AT_PORT & PID=\$!
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sleep 0.3
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echo -e 'AT+CGPADDR=1\r' > $AT_PORT
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sleep 2
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kill \$PID 2>/dev/null
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" 2>&1 | grep CGPADDR | grep -oE '[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+' | head -1)
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echo "Modem IP: $MODEM_IP"
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# 4. Configure interface
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ip link set eth1 up
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ip addr flush dev eth1
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ip addr add $MODEM_IP/32 dev eth1
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ip route add default dev eth1 metric 50
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# 5. Test
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ping -c 3 8.8.8.8
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```
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## Troubleshooting
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### AT Commands Not Responding
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**Symptoms:** No response to AT commands on any port.
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**Solutions:**
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1. Check modem is in Mode 40: `lsusb | grep 7126`
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2. If in Mode 41 (7127), AT commands won't work - need physical access to switch back
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3. Try different ports: ttyUSB0, ttyUSB1, ttyUSB2
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### ttyUSB3 Shows as Regular File
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**Symptoms:** `ls -la /dev/ttyUSB3` shows `-rw-r--r--` instead of `crw-rw----`
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**Fix:**
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```bash
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rm -f /dev/ttyUSB3
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mknod /dev/ttyUSB3 c 188 3
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chmod 660 /dev/ttyUSB3
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chown root:dialout /dev/ttyUSB3
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```
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### stty Errors
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**Symptoms:** `stty: /dev/ttyUSB1: cannot perform all requested operations`
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**Solution:** Don't use `stty`. Send AT commands directly with `echo` and `cat`.
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### No IP on eth1
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**Symptoms:** `ip addr show eth1` shows no inet address after DHCP attempt.
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**Solution:** RNDIS doesn't use DHCP. Configure IP manually from `AT+CGPADDR=1`.
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### ModemManager Not Detecting Modem
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**Note:** ModemManager doesn't work well with this modem in RNDIS mode. Use AT commands directly instead.
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### Signal Strength Interpretation
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| CSQ Value | dBm | Quality |
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|-----------|-----|---------|
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| 0 | -113 or less | No signal |
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| 1-9 | -111 to -95 | Poor |
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| 10-14 | -93 to -85 | Fair |
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| 15-19 | -83 to -75 | Good |
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| 20-30 | -73 to -53 | Excellent |
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| 31 | -51 or better | Excellent |
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| 99 | Unknown | - |
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## Kernel Modules
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These modules should be loaded for MBIM/QMI support (optional):
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```bash
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modprobe cdc-wdm
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modprobe cdc_mbim
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modprobe qmi_wwan
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modprobe cdc_ncm
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# Verify
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lsmod | grep -E '(cdc|qmi)'
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```
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## Files on Device
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| Path | Purpose |
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|------|---------|
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| `/usr/local/bin/connect-5g.sh` | Auto-connection script |
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| `/etc/init.d/5g-router` | OpenRC service |
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| `/var/log/5g-router.log` | Connection log |
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| `/etc/iptables/rules.v4` | Firewall/NAT rules |
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## Service Management
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```bash
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# Check status
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service 5g-router status
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# Restart connection
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service 5g-router restart
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# View logs
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tail -f /var/log/5g-router.log
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# Check if enabled at boot
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rc-update show | grep 5g-router
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```
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