Data usage too high... (yet another case)

I have read the previous discussions about this. How can I make Xfinity to fix my data usage report?

I have old Arris SB6121 modem without wifi and nothing wrong with it.

I have Google mesh system at home and can monitor data usage for each device / MAC address.

Xfinity data report goes linearly up to max every month last year. Looks like they are trying to make customers to buy unlimited plan.

We have used around 300gb this month (yesterday) and Xfinity usage was showing over 600gb.

Looking again today and their usage is showing over 700gb now and we used under 20gb yesterday. I'm logging our usage myself.

What to do? I already sent message to abuse@comcast.net

Nobody is exploiting our wifi. I would see the MAC address connected.