AT&T router/modem
What AT&T router do you have?
Unfortunately the fiber AT&T router can’t be set in true bridge/ip pass through mode. So when you use a router behind the isp router you can expect to have a double NAT among other issues.
The best solution is to use the AT&T router with the switch plugged into the router. If that doesn’t provide adequate WiFi coverage, disable the built in WiFi and add Ethernet wired Ubiquity Unifi access points where you need them(plugged into the switch). Like a U7 pro, U6+, U6 long range. Start with one of these and add more if you have a dead spot. A centrally located ceiling mounted U6 LR will handle 2 levels 1250sqft each for a 2500sqft house at least, probably more.