# FAQ-Fix Internet Connection Loss Error on Linux/Ubuntu 4G LTE USB Dongle (simcom A7680C)

> Source: https://maker.tdlogy.com/wiki-doc/faq-fix-internet-connection-loss-error-on-linux-ubuntu-4g-lte-usb-dongle-simcom-a7680c/

You plug a 4G LTE USB dongle (simcom A7680C-LNNV, Quectel, or other A76XX series) into your Linux computer, the ModemManager says registered, the LTE signal is full, but you can't access the Internet. The browser reports a DNS error, failing to load the website. Meanwhile, if you type your IP address straight (e.g. 8.8.8.8), the ping will still work.

You must use WiFi as a backup plan.

## Root causes

USB 4G LTE models use the RNDIS (Remote Network Driver Interface Specification) protocol — which creates a virtual Ethernet interface (for example) and automatically grants IP over the modem's internal DHCP.enxae0c29a39b6d

Normal Operating Procedure:

Modem 4G ──RNDIS──> Linux nhận interface `enx...`
                    DHCP từ modem → IP: 192.168.0.100/24
                    Gateway: 192.168.0.1 (chính modem)
                    Route mặc định qua modem

Problem: The modem grants IP and gateway via DHCP, but does not grant a DNS server to . Results:systemd-resolved

The system still uses a DNS server from WiFi (or Ethernet), so when WiFi is turned off, DNS dies completely.

## Rapid Diagnostic Test

### 1. Does the ModemManager receive the device?

mmcli -L

Expected results:

/org/freedesktop/ModemManager1/Modem/2 [SIMCOM INCORPORATED] A7680C-LNNV

### 2. Is the modem registered to the network?

mmcli -m 2

Important:

### 3. Check the IP and route

ip addr show
ip route show

You'll see the RNDIS interface (name) with the default IP and route.enx...192.168.0.100/24default via 192.168.0.1

### 4. DNS Audit

resolvectl status <tên_interface>

For example:

resolvectl status enxae0c29a39b6d

If you see and don't — that's why.Current Scopes: noneCurrent DNS Server

### 5. Confirm the modem can ping the IP

ping -c 3 -I enxae0c29a39b6d 8.8.8.8

If the ping is OK → the problem is DNS, not the connection.

## Fix Instructions

### Step 1: Check the DNS port on the modem

RNDIS modems typically run DNS relays at the gateway (192.168.0.1):

timeout 2 bash -c "echo > /dev/tcp/192.168.0.1/53" 2>/dev/null && echo "DNS port OPEN" || echo "DNS port closed"

If the port is open, → can use the modem itself as a DNS server.

### Step 2: Assign a temporary DNS (used immediately, lost after reboot)

sudo resolvectl dns enxae0c29a39b6d 192.168.0.1 8.8.8.8
sudo resolvectl default-route enxae0c29a39b6d yes

Review:

resolvectl query google.com

Expected results: google.com: 142.250... -- link: enxae0c29a39b6d

### Step 3: Assign a permanent DNS (hold after reboot)

#### Option A: Use systemd-resolved drop-in (recommended)

Create a configuration file:

sudo mkdir -p /etc/systemd/resolved.conf.d

sudo tee /etc/systemd/resolved.conf.d/modem-rndis-dns.conf << 'EOF'
# Persistent DNS config for SIMCOM 4G LTE RNDIS modem interface
[Resolve]
DNS=192.168.0.1 8.8.8.8
EOF

Systemd-resolved reboot:

sudo systemctl restart systemd-resolved

#### Option B: Use systemd service (automatically set DNS when the interface is up)

sudo tee /etc/systemd/system/resolvectl-modem-dns.service << 'SERVICEEOF'
[Unit]
Description=Set DNS for 4G LTE RNDIS modem
After=network.target sys-subsystem-net-devices-enxae0c29a39b6d.device
BindsTo=sys-subsystem-net-devices-enxae0c29a39b6d.device

[Service]
Type=oneshot
ExecStart=resolvectl dns enxae0c29a39b6d 192.168.0.1 8.8.8.8
ExecStart=resolvectl default-route enxae0c29a39b6d yes
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target
SERVICEEOF

Activation:

sudo systemctl daemon-reload
sudo systemctl enable resolvectl-modem-dns.service
sudo systemctl start resolvectl-modem-dns.service

Note: Replace it with the actual interface name on your device. The interface name can be checked by: enxae0c29a39b6dip link show | grep -v lo

### Step 4: Overall Inspection

# DNS đã hoạt động qua modem chưa?
resolvectl query google.com

# Ping tên miền đã OK chưa?
ping -c 2 google.com

# Tắt WiFi thử:
nmcli radio wifi off

# Kiểm tra lại:
ping -c 2 google.com

# Bật lại WiFi nếu cần:
nmcli radio wifi on

## System structure after fix

┌──────────────────┐
                    │   Internet 4G    │
                    │   (Nhà mạng)     │
                    └────────┬─────────┘
                             │ LTE
                    ┌────────▼─────────┐
                    │  SIMCOM A7680C   │
                    │  192.168.0.1     │
                    │  DNS relay: 53   │
                    └────────┬─────────┘
                             │ RNDIS (USB)
                    ┌────────▼─────────┐
                    │  enxae0c29a39b6d │
                    │  192.168.0.100   │
                    │  DNS: 192.168.0.1│
                    │        8.8.8.8   │
                    └────────┬─────────┘
                             │
                    ┌────────▼─────────┐
                    │  systemd-resolved │
                    │  + DefaultRoute  │
                    └──────────────────┘

## Important Notes

## References