Hi!
I have T-mobile prepaid for the 4g LTE data service - it costs me 25 bucks/month for 6GB/month.
When I log into my account on the T-mobile site, I can check “line usage” and it shows me data use for the month. I can also click through and it will show actual uses, and how much data per use. For example, my wife watched live view for 7 minutes or so, and I could see that afterwards it was around 23mb.
I think I can also log into the modem to see data use, but I’d have to be directly connected to it. Checking via T-mobile site I can check data from anywhere.