MANUAL NETWORK CONNECTION.

Thank you for patiently waiting. Upon checking here, we just need to choose an available country you are travelling.
 
Kindly follow the steps:

Go to the Home screen.
Choose Settings.
Scroll down.
Choose Cellular.
Choose Network Selection.
Turn off Automatic.
The Apple iPhone 12 is now searching for available networks.
Choose the available carrier on a country you are travelling network.