Nokia BGW320-505

Here's the HOW....

https://www.att.com/support/smallbusiness/article/smb-internet/KM1188700

Use the instructions for the Motorola and DHCPS-fixed (maybe someday AT&T will actually have instructions for current gateway models).

Disable the wifi radios on the BGW once you get your router working so as not to cause issues.

you dont , you must use the at&t provided modem which authenticates your account what you put behind it is up to you .