Alpine 5G Router - Raspberry Pi 5 + Fibocom FM350-GL

A complete 5G router setup using Alpine Linux on Raspberry Pi 5 with a Fibocom FM350-GL modem.

✅ Status: Working

The 5G modem is fully operational with CYTA Cyprus SIM card.

Documentation

File	Description
README.md	This file - overview and quick start
5G_MODEM_TROUBLESHOOTING.md	Complete modem configuration reference
configure_fm350_5g.sh	Configuration script for the modem

Hardware

• Board: Raspberry Pi 5
• Modem: Fibocom FM350-GL (USB ID: 0e8d:7126)
• OS: Alpine Linux v3.23.3
• SIM: CYTA Cyprus (APN: internet)

Network Architecture

Internet (CYTA 5G)
        │
        ▼
  FM350-GL Modem
   (RNDIS eth1)
        │
        ▼
  Raspberry Pi 5
   Alpine Linux
        │
        ▼
   eth0.100 VLAN
   (192.168.1.1)
        │
        ▼
   LAN Clients

Quick Start

1. Install Required Packages

# Enable community repository
sed -i 's|#.*community|http://mirrors.neterra.net/alpine/v3.23/community|' /etc/apk/repositories
apk update

# Install packages
apk add modemmanager dnsmasq iptables libmbim-tools qmi-utils

2. Configure and Connect Modem

# Set APN for CYTA
cat /dev/ttyUSB1 & CAT_PID=$!
sleep 0.3
echo -e 'AT+CGDCONT=1,"IP","internet"\r' > /dev/ttyUSB1
sleep 2
kill $CAT_PID

# Activate connection
cat /dev/ttyUSB1 & CAT_PID=$!
sleep 0.3
echo -e 'AT+CGACT=1,1\r' > /dev/ttyUSB1
sleep 3
kill $CAT_PID

# Get modem IP
cat /dev/ttyUSB1 & CAT_PID=$!
sleep 0.3
echo -e 'AT+CGPADDR=1\r' > /dev/ttyUSB1
sleep 2
kill $CAT_PID
# Note the IP address returned (e.g., 10.156.167.104)

3. Configure Network Interface

# Replace MODEM_IP with the IP from AT+CGPADDR=1
MODEM_IP="10.156.167.104"

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 connectivity
ping -c 3 8.8.8.8

4. Setup NAT for LAN

# Enable IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

# Configure NAT
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
iptables -A FORWARD -i eth0.100 -o eth1 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0.100 -m state --state RELATED,ESTABLISHED -j ACCEPT

# Save rules
iptables-save > /etc/iptables/rules.v4

Key Configuration Details

Modem USB Modes

Mode	USB ID	Description	Status
40	0e8d:7126	RNDIS mode	✅ Working
41	0e8d:7127	Extended mode	❌ AT broken

Important: Stay in Mode 40 - AT commands work on /dev/ttyUSB1.

Working AT Commands

Command	Description
AT	Test communication
AT+CSQ	Signal strength
AT+CGDCONT=1,"IP","internet"	Set APN
AT+CGACT=1,1	Activate connection
AT+CGPADDR=1	Get assigned IP
AT+CGCONTRDP=1	Get DNS servers
AT+GTUSBMODE?	Check USB mode

CYTA Cyprus Network Info

• APN: internet
• DNS Primary: 195.14.130.220
• DNS Secondary: 195.14.154.100

Services

The following services are configured to start on boot:

# Check service status
rc-status

# Services enabled:
# - dnsmasq (DHCP/DNS)
# - iptables-restore (firewall rules)
# - 5g-router (connection script)

Files on Device

Path	Purpose
/usr/local/bin/connect-5g.sh	Startup connection script
/etc/init.d/5g-router	OpenRC service
/etc/init.d/iptables-restore	Firewall restore service
/etc/iptables/rules.v4	Saved firewall rules
/etc/dnsmasq.conf	DHCP configuration
/etc/network/interfaces	Network configuration
/var/log/5g-router.log	Connection log

Troubleshooting

Modem not responding to AT commands

1. Check modem is in Mode 40: lsusb | grep 7126
2. Use /dev/ttyUSB1 for AT commands
3. Don't use stty - send commands directly

DHCP not working on eth1

This is normal - RNDIS mode doesn't provide DHCP. Configure IP manually using the address from AT+CGPADDR=1.

Connection drops

Re-run the connection script:

/usr/local/bin/connect-5g.sh

Or restart the service:

service 5g-router restart

Verification Commands

# Check modem
lsusb | grep -i fibocom

# Check network
ip addr show eth1
ip route show

# Test connectivity
ping -c 3 8.8.8.8
ping -c 3 google.com

# Check NAT rules
iptables -t nat -L -n -v

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