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What is the best modem / router combo you recommend. The home is approx 3500 SQFT.
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.