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.
Slide your finger down the display starting from the top edge of your phone.
Press the settings icon.
Press Mobile data.
Press Network operators.
Select one of the following options:
Select network manually, go to 2a.
Select network automatically, go to 2b.
Press Search networks.
Press the required network.
The display will tell you if you have access to the network you've chosen.