Internet Usage Monitor

I got an alert from xfinity that our home xfinity internet WIFI usage has exceeded the limit of 1229GB. This seemed improbable for us, since our usage pattern did not change. So we closely monitored our usage today. We almost disconnected everything and set our phone to low data mode, but the usage is still climbing up like crazy (about 20GB per hour, which is impossible, as no one is streaming or playing games). Does anyone know if there is any way to check which device is using the most data?

We also kind of suspect there might something wrong with the xfinity's internet usage monitor. We tried to connect to a live agent using the xfinity assistant but never succeeded.

This chart is what I got from internet.xfinity.com. There is no data for most of the last 24 hours? When I checked at around 3 pm today this was an empty chart with no data at all. How is that possible? Does it suggest some bugs with xfinity's usage monitor??