Samsung Galaxy S21 Ultra
It's not possible to use my phone's internet connection
Solution 1 of 4: Data roaming is turned off
You can limit your data usage when abroad by turning off data roaming. You'll then not be able to access the internet using the mobile network when abroad. You can still use Wi-Fi even though data roaming is turned off.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Connections.
Press Mobile networks.
Press the indicator next to "Data roaming ‹SIM›" to turn the function on or off.
Press the Home key to return to the home screen.