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 two fingers downwards starting from the top the screen.
Tap the settings icon.
Tap Mobile network.
Tap Carrier and wait while your mobile phone searches for networks.
To select a network automatically, tap the indicator next to "Choose automatically" to turn on the function.
To select a network manually, tap the indicator next to "Choose automatically" to turn off the function.