I think I found the culprit... I'll try to translate the menus from my italian iPhone in English, anyway for me the problem was whatsapp, with weekly conversation backup on icloud.
The problem is that even if you disable Whatsapp access to mobile data, the backup still goes on because it is seen as a system services and uses iCloud Drive. Of course you could disable iCloud Drive access to data network but here comes the issue: you can disable/enable data in two places: Settings/Cellular where you enable/disable data for all the apps, there's "iCloud Drive". Then if you go in Settings/iCloud/iCloud Drive there's a switch for "access to cellular data" which is INDEPENDENT from the one you set in the other section of the preferences. I think this is a bug and it was probably the reason why it consumed all that data.