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 networks"
Press the menu icon.
Press Wireless & networks.
Press Mobile networks.
Activate or deactivate mobile data
Press Data enabled to activate or deactivate mobile data.
When the box next to the menu item is ticked (V), mobile data is activated.
Return to the Home screen
Press the Home key to end the session and return to the Home screen.