In normal circumstances, Firsty always select the best possible network based on your location. However, in some cases, you might find it more comfortable to choose a network yourself. In that case, you can follow these steps:
Selecting Network Operator Manually on iOS
Go to Settings.
Select Mobile Service (or Cellular on some iPhone devices).
Tap on the installed eSIM.
Tap Network Selection.
Disable the automatic selection.
Select from the list of network providers that appears.
Selecting Network Operator Manually on Android
Go to Settings.
Tap Network & Internet.
Select SIMs (or SIM cards / Mobile networks).
Tap your Firsty eSIM.
Tap Select Carrier ('Network operator' in some cases).
Toggle the automatic selection off.
Select from the list of network providers that appears.