You can avoid unintended data traffic by deactivating mobile data. No connection to the Internet via the mobile network can then be established. You can still use WLAN even though mobile data is deactivated.
Find "mobile network"
Slide your finger left on the display.
Press mobile network.
Activate or deactivate mobile data
Press the field below "Data connection".
Press on or off.
Return to the Home screen
Press the Home key to end the session and return to the Home screen.