My data usage jumped a ridiculous amount last month and I'd love to monitor the data use at the device level. Anyone have tips on how to do this with an Arris router?

www.wikihow.com/Monitor-Network-Traffic

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.

I have no answer...but experiencing the same thing.  I'd like to be able to track usage by device in my home...so that I have an idea of which family member is causing it ....or is it even accurate.

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.