I have the same provider and have been able to track and manage usage by installing software at the router. I'm using Gargoyle router management software https://www.gargoyle-router.com. If you have a compatible router, I'd suggest trying that. The software not only measures usage, but allows setting of quotas for usage. The usage totals from Gargoyle are within a couple of percent of those from the ISP, as there is some traffic that hits the modem that does not make it to the router (filtered at the modem).