You may have the same problem we did. It is easy to monitor basic info using Cox. Problem was change in YouTube policy to default to highest definition setting available for the video unless you do something different with your YouTube settings. Made our data usage sky rocket.
It seems to be accurate, but Cox only offers a daily tracking of total usage at https://www.cox.com/internet/mydatausage.cox. Video is the overwhelming culprit due to the large amount of data transfer needed and the higher the resolution, the worse it gets. We stream a fair amount of YouTube but didn't and don't have a usage problem as long as we avoid high definition that really isn't needed for computer viewing.
Not sure if you've found the definitive answer to your question, but if you have the time and money and your modem and router are separate, you could purchase a small 5-port managed switch (i.e. Netgear GS105Ev2) that has port mirroring and install it between the modem and router WAN port. Configure the switch to mirror the port connected to the router WAN port to one of the remaining available ports on the switch. Connect an ethernet cable from that port to an ethernet port running in "promiscuous mode" on a PC or Laptop. Install network sniffing/analyzing software on the PC or Laptop such as Wireshark (free) to capture and analyze the packets. Good luck.
Mine jumped significantly and when I did start tracking it from my router, the discrepancy between my router and Cox's Data Meter was 312%. They are blaming the tracking tool on my router; but according to my router documentation it should be 10 - 11% (mostly what we used to call "handshakes"). Not over 312%. Cox wasn't even interested in investigating it.