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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 "Advanced"
Slide your finger downwards starting from the top of the screen.
Press the settings icon.
Press Network & Internet.
Press Mobile network.
2. Select network manually
Press the indicator next to "Automatically select network" and wait while your phone searches for networks.
Press the required network.
3. Select network automatically
Press the indicator next to "Automatically select network".
4. Return to the home screen
Press the Home key to return to the home screen.