If you have a pano modem you can use the pano app. If you have a higher end router, you can turn on data monitoring in the router settings and it will monitor each device.
Info - Device streaming 4k video, can use as much as 8gb an hour.
Some gaming systems if you reload the downloaded games, can use a WILD amount of data.
If you have an android phone and you have one drive installed, verify the settings, it COULD be backing up the entire phone daily and that can use over 100gb daily in some cases. (Another users in these forums, found that exact issue).
If you have Amazon echo or dot, they can go into a loop listening and translating all conversations, and tv show you are watching and they can use as LOT of data.
If you have kids, make sure they didn't give the WiFi password to a neighbor, (also something another forum poster found out).
Hope some of this helps.
If not let me know, I can help provide more info.