With Att fiber, I had to use their gateway or not have internet. You can set it to passthrough mode but I didn’t really trust that even with my own router and their wifi disabled.

On Comcast/xfinity copper 2.5, I am 100% using my own modem and router. They kept pushing me to get a streaming box, thermostat, camera, whatever the fuck else and it took several steps to get just the basic service.