Hi,
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.
EB.