Overages we can't figure out

I see a lot of past messages about overages and people trying to figure out where they are suddenly hitting such high numbers after not changing their usage habits.  Does anyone have knowledge of how this metering actually works?  Two of us in this household are scratching our heads trying to find out why we are suddenly going over.  Device count has not increased.  Habits have not changed.  In fact, we've backed it off and just got our second in a row overage notice.  The only thing that has changed is a new up-to-date cable modem because Cox bugged us to stop using the previous one.  Could that be doing it?  If so, man that's a kick in the pants.  At the very last moment we went over our 1024Gb limit by 2GB.  1026Gb of our 1024Gb and now we get to pay an overage fee.  How convenient.  My $300 router doesn't have a built in mechanism to meter each device's usage.  Does anyone have one that does and does so without having to establish a presence on the router manufacturer's website?  

Frustration building.