1. Access the IEOS web interface and go to [Network > Cellular]. 2. Enable "Dual SIM Enabled". 3. Select "Main SIM" (SIM1 by default). 4. Set "Max Number of Dials" (times before switching to the other SIM). 5. Save the setting— the gateway will auto-switch to SIM2 if SIM1 dial fails (e.g., due to arrears).