Please check whether your mobile device is using data in the background. This can be caused, for example, by open programs or update processes.

If you use the SIM card in a smartphone, different applications may consume data in the background. On e.g. Android devices you can check the apps under Settings -> Networks -> Data consumption. Set the apps with a high data consumption (forbid background activity) so that no more data will be consumed.

Here are some more tips and instructions: https://www.heise.de/tipps-tricks/Datenvolumen-unter-Android-sparen-die-besten-Tipps-4035610.html