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You can set your phone to select a network automatically or you can select a network manually. If you select a network manually, your phone will lose network connection when the selected network is out of reach.
1. Find "SIM settings"
Slide your finger downwards starting from the top of the screen.
Press All settings.
Press Network & wireless.
Press Mobile & SIM.
Press SIM settings.
2. Select network
Press Search for networks.
If you want to select a network automatically, press automatic.
If you want to select a network manually, press the required network.
3. Return to the home screen
Press the Home key to return to the home screen.