I ditched AT&T's router for UniFi using one setting they don't advertise

I have had AT&T Fiber for over seven years at this point. While I enjoy the service and have had very few reliability problems with it, I really just want to use my own router. AT&T doesn't allow you to use anything but its provided gateway, and it's always annoyed me. Here's how I got around their rules and started using my own router with AT&T anyway.