iPad using too much data on WiFi?
Much of iCloud will synchronize when you are connected to WiFi and not cellular data.

Check in the settings app and be sure to turn off iCloud backups (or restrict severely the parts of the data that backup). Also, look at things like iCloud photo library, podcast apps, My Photo Stream and others that wait until you connect to WiFi to avoid burning cellular data.

Can you take it to a proper WiFi temporarily to see if after 30 minutes whatever backup and sync are needed complete and then you will have lower usage going forward?