1. Log in to the web interface > Go to [Network > Dialup Port]. 2. Enable "Dual SIM Enable". 3. Select "Main SIM" (SIM1 by default). 4. Set "Max Number of Dial" (times before switching to the backup SIM, default 5). 5. Save settings— the router will auto-switch to SIM2 if SIM1 fails to dial (e.g., arrears, poor signal).