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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.
Find "Network operators"
Press the settings icon.
Select one of the following options:
Select network manually, go to 2a.
Select network automatically, go to 2b.
2a - Select network manually
The display will tell you if you have access to the network you've chosen.