Do I need my ISP router?

Hey everyone! I'm new to the home networking hobby but am really digging it. Currently I've got all my routing handled by a UDM Pro and a Switch Lite 8. I've just ordered a 24 POE to take over for the 8, can't wait to install it!

I'm curious though, about the way my ISP has me set up. I've got AT&T Fiber, so I've got a fiber converter box thing in the garage that comes from the service. That's then attached to a modem/router supplied by AT&t. I've got all the networks turned off so there's no interference with my AP's, but I wonder if I need that box at all? Could I just plumb the ethernet from the converter box into the UDM Pro?