You can set your mobile phone to find a network automatically or you can choose a network manually. If you select a network manually, the network connection will be lost if you're out of range of the selected network.
Slide your finger downwards starting from the top of the screen.
Tap the settings icon.
Tap Mobile networks.
Tap Network operators.
Tap Search networks and wait while your mobile phone searches for networks.
Tap the required network.
If you want to select a network automatically, tap Select automatically.