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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 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.