Sudden, huge increase in data usage

We typically use ~500 GB/month, if even that. In January, in a single day, we received email notices that we had reached 75% (1:00 am, not yet online), then 90% (7 am, not yet online), then 100% (1:00 pm) of the 1.2TB limit. After frustrating calls with support (they did not understand that the reported data usage was not possible), I gave in and got the unlimited data plan to avoid fees or being throttled. That was six days ago, and we are now at 3.2TB usage and climbing.

There has been no change in our use at all, no long sessions of streaming/gaming or long video calls, and no unusual devices connected to our wifi. A quick google suggests that there's some issue with their data usage reporting/calculating, and support has been useless thus far. I need to DM with an actual person who can compare screenshots of our router usage data with xfinity's and can see that there is an error in xfinity's data usage report. I don't want to pay for an unlimited data plan that I do not need, as we have never come close to the 1.2TB limit before.