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What is the best modem / router combo you recommend. The home is approx 3500 SQFT.
You'll have to first chose a modem (DOCSIS) compatible on the Cox network. Then choose if you want a separate modem and separate router.
At 3500 sq feet, tou are going to either need to purchase a MESH system, like the Google - https://www.amazon.com/Google-Wifi-System-Router-Replacement/dp/B08GG9CMLR
Or get the Cox Pano modem, and I would get at least 2, if not 3 of the Cox Pods. The downside to the pods, you have to purchase them outright and they ONLY work with the pano modem.
Barring that, getting a power line adapter, and at least 2 Ethernet/WiFi ends, means you can use them to setup mesh networking AND, you will have an ethernet port on each of the ends, so you can use it let's same to provide ethernet to your gaming system, or TV, and then the WiFi provides mesh support as long as you have given it the same SSID and WPA key aws your main WiFi Router has. I use a tp link and it works extremely well.
Building on what Darkatt said, it really depends on a) your network needs b) what connectivity is readily accessible.
I'd plan on a DOCSIS 3.1 Modem.
If you want to really understand and control your system, I'd recommend separate devices that you purchase outright.
How many devices do you need to connect? How many need to support higher throughput, such as 4K streaming?
Are you mainly focused on connectivity, or more integrated smart home integration.
Can you readily run (or have in place) ethernet? Coax that will support MOCO?
What's a reasonable budget?
Are you comfortable with web based router controls, such as Google's, or do you want everything on a local device?
I used to use strictly Netgear but have become very satisfied with Arris modem and Google WiFi combos, in part because of the ease of integration with my fairly extensive "smart home" integration. I have a Motorola modem right now, just because I needed something in a hurry (like less than an hour) last time I worked on a plan change with Cox, and Best Buy didn't have the Arris I wanted in stock. I use first gen Google wifi with ethernet backhaul.
When I started out, I had a blend of Amazon, Google, Hue, Samsung Smart Things ecosystems going on, along with a wider range of devices. My focus was on diversifying and experimenting while the systems matured. All of the Amazon gear has been retired. Hue will be in the near future. I'm not fond of the Samsung Smart Things, but it supports some integrations that aren't natively supported with Google Home.
Nice input! Speaking of inputs, if you don't have a wireless printer or want to network an external hard drive, a couple USB ports on the router would be useful. If connecting a printer via USB, look for a router with print-server software.