CM1000v2 Internet Conundrum

follow Kitsap advice. The first part is to determine the bottleneck in your speeds. Test it directly connected to the modem. It should get 1 gig (actual 940mbps ish). if its not, then get a copy of the cable connections (shows upstream/downstream values) and the event logs and post them here. That helps us to check the line as Cox's first response is always "its not our line, Its fine. Must be your gear". 

You can check your connections yourself (and its recommended).Start with removing any amplifiers, signal attenuators, or splitters from the coax.
From there check the line for kinks, damage, moisture in the line.
Check the connectors for improperly made ends, foil touching the copper coax line, loose connections, bad/old/cheap connectors, or corroded connections. Replace them if you do.
If you can, simply connect the modem right where the coax comes into the home. This prevents wiring in the home from being the issue. And some ISP’s charge if the wiring issue is in the home. So this helps prevent this.