While you will need to keep using the gateway provided by AT&T, however, you will be able to set up your own router into the AT&T gateway and avoid the wireless network on the gateway of AT&T.
The Bridge Mode Configuration works by disabling the Network Address Translation (NAT) feature on the modem and keep in check any conflict of IP address by making the router to function as a DHCP server.
This configuration feature allows you to connect the two routers (Your router and the one provided by AT&T) to connect on a single network while avoiding any performance hindrance.
To use your own router with AT&T Fiber, you will need to configure the AT&T fiber router using the Bridge Mode configuration.
Whatever may be the reason you have, you can easily use your own router with AT&T by following these steps. It can also be a way to avoid router charges (if any). Maybe the router you have has a better configuration than the one provided by AT&T or the range provided by AT&T router is not adequate enough to provide signals to your big house. There can be numerous reasons that you may want to use your personal router to connect with AT&T Fiber. Guide to Use Your Own Router with AT&T Fiber
Getting a True Bridge Mode Connection With AT&T Fiber Router/Modem.Guide to Use Your Own Router with AT&T Fiber.