I must be dumb, but I don't understand how to do this.
Have 2 routers??
Very reasonable question. It is confuzing.
The best way to do a home network, or buis network, IMHO,, is to do the following. BUT. This requires some configuration of devices and more boxes. It is also fairly complex.
The chain of devices:
Cable MODEM. This is a modem ONLY. No router functions. Its just a interface to the cable system. For example ArrisS33 or SB8200.
https://www.surfboard.com/products/cable-modems/s33/
Router. This does a bunch of stuff providing the interface of all your devices on your network with the world outside thru the modem. This *might* have a wifi access point in it. I think its best to not have wifi in the router. For example.
https://mikrotik.com/product/ccr2216_1g_12xs_2xq
Switch. Hooks a bunch of devices to one connection that goes to the router. It also does some isolation of packets between ports. A switch can isolate packets locally between devices connected to the same switch. This is where its used a lot in audio. A lot of different audio tuned switches are discussed at Whats Best.
Access point. Provides wifi. This is a dedicated device just for that. Like Aruba or Ubiquiti. For example
https://www.arubanetworks.com/assets/ds/DS_AP650Series.pdf
To hook all this up is not trivial. If you want to do multigig everything needs to be at least 2.5 Gbps. Things like SFP+ modules, single mode fiber, SFP+ Direct connect, What types of ethernet to use and how all this needs to be done with audio in mind is complex. Each system will be different.
Cable companies want you to get a gateway. A do everything in one box device. While incredibly simple, it has a number of limitations. You cant do each function as well. A powerful router can never be stuffed into a gateway. Plus you cant upgrade the router to a new one as needed. Wifi in a gateway cant be as good as these separate companies because companies like Aruba have patents covering key stuff and technology. You can put access points all over the house and provide a far better wifi experence then using repeaters.
Stuffing everything into one box degrades performance in a number of ways as discussed here like turning off wifi to get better sound.
While a gateway is incredibly simple, a one box solution has limitations.
Good to befriend a network guy who you can get excited about doing some exotic networking. Aruba Instant is pretty easy, Mikrotik is medium level. Ubiquity has some easy solutions, but not always the best performance. A lot of this stuff is on ebay and you can get things used cheap and just experiment.