Alpine_5G/5G_MODEM_TROUBLESHOOTING.md

Fibocom FM350-GL 5G Modem Configuration Guide

Complete reference for configuring the Fibocom FM350-GL modem on Alpine Linux.

Working Configuration

Setting	Value
Modem	Fibocom FM350-GL
USB ID	0e8d:7126 (Mode 40)
AT Port	/dev/ttyUSB1
Network Interface	eth1 (RNDIS)
APN	internet (CYTA Cyprus)

AT Command Reference

Sending AT Commands

The FM350-GL doesn't need stty configuration. Send commands directly:

# Pattern for sending AT commands
cat /dev/ttyUSB1 & CAT_PID=$!
sleep 0.3
echo -e 'YOUR_COMMAND\r' > /dev/ttyUSB1
sleep 2
kill $CAT_PID 2>/dev/null

Essential Commands

Command	Purpose	Example Response
AT	Test communication	OK
AT+CGMI	Manufacturer	Fibocom Wireless Inc.
AT+CGMM	Model	FM350-GL
AT+CSQ	Signal strength	+CSQ: 7, 99 (7 = moderate)
AT+CREG?	Network registration	+CREG: 0,1 (1 = registered)
AT+CEREG?	LTE/5G registration	+CEREG: 0,1
AT+CCID	SIM card ID	+CCID: 893570...

APN Configuration

# Set APN (CYTA Cyprus uses "internet")
echo -e 'AT+CGDCONT=1,"IP","internet"\r' > /dev/ttyUSB1

# Verify APN is set
echo -e 'AT+CGDCONT?\r' > /dev/ttyUSB1
# Response: +CGDCONT: 1,"IP","internet",""...

Connection Management

# Activate PDP context (start data connection)
echo -e 'AT+CGACT=1,1\r' > /dev/ttyUSB1
# Response: +CGEV: ME PDN ACT 1, OK

# Get assigned IP address
echo -e 'AT+CGPADDR=1\r' > /dev/ttyUSB1
# Response: +CGPADDR: 1,"10.156.167.104",""

# Get full connection details (DNS, MTU)
echo -e 'AT+CGCONTRDP=1\r' > /dev/ttyUSB1
# Response includes DNS servers

USB Mode Control

# Check current USB mode
echo -e 'AT+GTUSBMODE?\r' > /dev/ttyUSB1
# Response: +GTUSBMODE: 40

# Available modes
echo -e 'AT+GTUSBMODE=?\r' > /dev/ttyUSB1
# Response: +GTUSBMODE: (40,41)

# Switch modes (requires modem reset)
echo -e 'AT+GTUSBMODE=40\r' > /dev/ttyUSB1

# Reset modem
echo -e 'AT+CFUN=1,1\r' > /dev/ttyUSB1

USB Modes

Mode	USB Product ID	Description	AT Commands
40	0e8d:7126	RNDIS mode	✅ Work on ttyUSB1
41	0e8d:7127	Extended mode	❌ Don't work

Important: Stay in Mode 40 for reliable AT command access.

Network Interface Configuration

Why DHCP Doesn't Work

The RNDIS interface (eth1) does not provide DHCP. The modem manages the connection internally. You must:

1. Get the IP from the modem via AT+CGPADDR=1
2. Configure the interface manually

Manual Configuration

# Get IP from modem (replace with actual value)
MODEM_IP="10.156.167.104"

# Configure interface
ip link set eth1 up
ip addr flush dev eth1
ip addr add $MODEM_IP/32 dev eth1
ip route add default dev eth1 metric 50

# Test
ping -c 3 8.8.8.8

CYTA Cyprus Settings

Setting	Value
APN	internet
DNS Primary	195.14.130.220
DNS Secondary	195.14.154.100
Network Name	Vodafone

Complete Connection Sequence

#!/bin/sh
# Full connection sequence for FM350-GL with CYTA SIM

AT_PORT="/dev/ttyUSB1"
APN="internet"

# 1. Configure APN
cat $AT_PORT & PID=$!; sleep 0.3
echo -e "AT+CGDCONT=1,\"IP\",\"$APN\"\r" > $AT_PORT
sleep 2; kill $PID 2>/dev/null

# 2. Activate connection
cat $AT_PORT & PID=$!; sleep 0.3
echo -e "AT+CGACT=1,1\r" > $AT_PORT
sleep 3; kill $PID 2>/dev/null

# 3. Get IP address
MODEM_IP=$(timeout 5 sh -c "
    cat $AT_PORT & PID=\$!
    sleep 0.3
    echo -e 'AT+CGPADDR=1\r' > $AT_PORT
    sleep 2
    kill \$PID 2>/dev/null
" 2>&1 | grep CGPADDR | grep -oE '[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+' | head -1)

echo "Modem IP: $MODEM_IP"

# 4. Configure interface
ip link set eth1 up
ip addr flush dev eth1
ip addr add $MODEM_IP/32 dev eth1
ip route add default dev eth1 metric 50

# 5. Test
ping -c 3 8.8.8.8

Troubleshooting

AT Commands Not Responding

Symptoms: No response to AT commands on any port.

Solutions:

1. Check modem is in Mode 40: lsusb | grep 7126
2. If in Mode 41 (7127), AT commands won't work - need physical access to switch back
3. Try different ports: ttyUSB0, ttyUSB1, ttyUSB2

ttyUSB3 Shows as Regular File

Symptoms: ls -la /dev/ttyUSB3 shows -rw-r--r-- instead of crw-rw----

Fix:

rm -f /dev/ttyUSB3
mknod /dev/ttyUSB3 c 188 3
chmod 660 /dev/ttyUSB3
chown root:dialout /dev/ttyUSB3

stty Errors

Symptoms: stty: /dev/ttyUSB1: cannot perform all requested operations

Solution: Don't use stty. Send AT commands directly with echo and cat.

No IP on eth1

Symptoms: ip addr show eth1 shows no inet address after DHCP attempt.

Solution: RNDIS doesn't use DHCP. Configure IP manually from AT+CGPADDR=1.

ModemManager Not Detecting Modem

Note: ModemManager doesn't work well with this modem in RNDIS mode. Use AT commands directly instead.

Signal Strength Interpretation

CSQ Value	dBm	Quality
0	-113 or less	No signal
1-9	-111 to -95	Poor
10-14	-93 to -85	Fair
15-19	-83 to -75	Good
20-30	-73 to -53	Excellent
31	-51 or better	Excellent
99	Unknown	-

Kernel Modules

These modules should be loaded for MBIM/QMI support (optional):

modprobe cdc-wdm
modprobe cdc_mbim
modprobe qmi_wwan
modprobe cdc_ncm

# Verify
lsmod | grep -E '(cdc|qmi)'

Files on Device

Path	Purpose
/usr/local/bin/connect-5g.sh	Auto-connection script
/etc/init.d/5g-router	OpenRC service
/var/log/5g-router.log	Connection log
/etc/iptables/rules.v4	Firewall/NAT rules

Service Management

# Check status
service 5g-router status

# Restart connection
service 5g-router restart

# View logs
tail -f /var/log/5g-router.log

# Check if enabled at boot
rc-update show | grep 5g-router