I’m not sure about this one, but I recently installed LibreNMS (an open source nms) on my home network and it does a decent job keeping track of traffic on my ubiquiti edgerouter. It uses snmp so should work with most routers, I imagine. I specifically set it up to tell me when I use more than a certain amount of data in a day to avoid data caps. It was a bit fiddly to set up, but kind of fun.