1. Log in to the web interface > Go to [Network > Cellular]. 2. Check "Dual SIM Enable" to activate the feature. 3. Select "Main SIM" (SIM1 by default). 4. Set "Max Number of Dial" (default 5 — the device switches to SIM2 if SIM1 fails to dial). 5. Click "Apply & Save" — the gateway will auto-switch to the backup SIM for uninterrupted connectivity.