Best modem/router for Fiber

you have to use the at&t gateway regardless of what you want to do. You have two options for your equipment:

1) just get wireless access points and use the gateway's routing
2) set the gateway to "IP Passthrough" mode and use your own router. Their gateway will still be performing NAT and inspecting packets so you add about 1ms delay when you use your own router. You will still have the session limits if you use your own router. There will be a couple ports you just can't use because the gateway uses them.