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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.
Select network manually, go to 2a.
Select network automatically, go to 2b.
2a - Select network manually
Press the indicator next to "Automatic" until the function is turned off.
Press the required network.
Your display will tell you if you've access to the network you've chosen.