Instructions:
  1. Log into your device's WebUI.
  2. Navigate to Network → Interfaces.
  3. Press Edit on the mobile interface.  4. Untoggle / Toggle Auto APN. 


Legacy WebUI

Auto APN is enabled by default. But if you have just inserted the SIM card you might need to restart (RUT9XX/RUT2XX) routers modem first (Status/Mobile/Mobile Information).

Instructions:
  1. Log into your device's WebUI.
  2. Navigate to Network → Mobile.
  3. Uncheck / Check Auto next to APN.