Data leak or equipment issues

Cox probably won't bother to tell you that Arris equipment has many well known unpatched security flaws, in one case a backdoor allowing full network access. Since you sound like you don't use much network bandwidth and then magically you started using so much you Max out your limit, well that sounds like an equipment issue related to an owned device using your high speed connection. Cox owns the responsibility for modem security since you are not allowed to upgrade it for any purpose including to secure it.Tom's hardware has a write up that might help www.google.com/url .  You should not have to be this far in the weeds, but what you are describing sounds like malware you can't see on recent equipment additions you don't control. Routers, modems, and other Internet of things devices are that hot hacker Target at the moment; the bandwidth has multiple uses for hackers.