Sometimes called a 'manual roam', you can do this in your phone settings.
Generic method
Go to 'Settings'
Then go to 'mobile data/wireless networks'
Find something like 'Network selection'
Choose the network you want to be on
Manufacturer specific guides
Apple
Apple
Go to 'Settings'
Click on 'Cellular' (or 'Mobile Service')
Tap on 'Network Selection'
Turn 'OFF' the Automatic toggle switch
Wait for the list of available networks to appear
Then choose a network from the list of Available networks
Android
Android
Go to 'Settings'
Select 'Connections'
Select 'Mobile networks'
Then 'Network operators'
Toggle 'OFF' Select automatically
Then choose a network from the list of Available networks
Windows
Windows
Go to 'Settings'
Then 'Network & Internet', and then 'Cellular & SIM'
Click on 'SIM settings'
Click 'Network selection'
Toggle 'OFF' Select automatically
Tap 'Search for networks'
Then choose a network from the list of Available networks
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