FWIW, I’m on AT&T Fiber in Texas. I have port forwarding working with a BGW 320-500. I did not need to add a second router, use passthrough/bridge mode, etc.
See this thread (also linked above): Changed ISP - No Remote Access with AT&T BGW 320-500
Those screenshots are from my BGW 320-500 and Plex Media Server running on my Windows 10 PC.
The post uses 32400 for the external port. If you want to use a different port, change the Global Port Range.
The Host Port is always 32400.
I loaded Plex on my PC to grab those screenshots. My real Plex server runs on a different external port.
