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" below "Wireless & networks".
Press Mobile network.
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 a network manually
Press Choose automatically until the function is deactivated.
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 Choose automatically until the function is activated.
Return to the Home screen
Press the home icon to end the session and return to the Home screen.