I just switched to a highspeed DOCSIS 3.1 version and am very happy. If you have the capability, you might consider an upgrade, as the speed increases are huge. Anyway, that's a discussion for a later point.
First, have you recycled BOTH the modem and the router? Give that a try and just make sure all cables are secure.
If that doesn't work, do you have an ethernet cable? If so, can you do a direct connect of one your laptops into the modem using the ethernet capable? Check the speeds etc then to see if things improve. If it does, then you probably do need to consider a new router, but I would doubt it. Try also connecting the laptop to the router if there is a free ethernet port there. This helps you isolate where the problem MIGHT be.
If that doesn't work, have you spoken to Cox? They will probably say that, because it's your own equipment, there's not much they can do. However, they should still be able to some codes on their end to rule out any problems at Cox's end.
ON EDIT: I just saw that Cox owns the modem. Big time get them to help you troubleshoot!
Finally, here's Netgear's site, with a similar issue. Invariably, they always say to get a new router. I am too cynical to fall for that, as I have never had a router or modem fail. https://community.netgear.com/t5/General-WiFi-Routers-Non/slow-connection-Netgear-N600-router/m-p/2032201 .
If all that doesn't work, I never give up....there is an issue, and it can be fixed somehow. We just need to figure out the "somehow"!