Lose internet on ethernet connection with AT&T but not T-Mobile

I've narrowed this problem down, but can't figure it out.  I've simplified my setup by running an ethernet cable direct from my internet gateway to my docking station for my Dell laptop running Windows 11.  When I plug the ethernet cable into the AT&T gateway it connects, then about 5 minutes later the internet disconnects.  When I plug the ethernet cable into the T-Mobile Home Internet modem it connects and stays connected.

I've rebooted everything, numerous times.  I've been working on this for days.  Any ideas?  Thanks.