You can set your mobile phone to select a network automatically or you can select a network manually. If you select a network manually, your mobile phone will lose connection to the network when the selected network is out of range.
Find "Network operators"
Press More settings.
Press Mobile networks.
Press Network operators.
Select one of the following options:
Select a network manually, go to 2a.
Select a network automatically, go to 2b.
2a - Select network manually
Your mobile phone will search for networks within range.
Press the required network.
Your display will tell you if you have access to the network you've selected.
2b - Select a network automatically
Press Select automatically.
Return to the Home screen
Press the Home key to end the session and return to the Home screen.