Make sure you go to your services and check if there's anything that could be downloading or uploading the data enabled.
If you're not sure - go to Status > Services and take a screenshot and post it here.
After that, go to System > Setup Wizard > RMS. Sometimes RMS communication, even when not used could eat up a lot of data. Try disabling it.
Also, check if your access points are configured and you're not just sharing free internet with everybody, might sound absurd, but sometimes this happens.