@Liverpool20 

Thanks for the advice.  Yes, the WAN IP address on the AX11000 was the same as the AT&T "fiber gateway" device.  I tried everything I could think of and worked with an AT&T specialist for a long while, then also rebooted both systems multiple times after making changes. The best we could do was to get the Wi-Fi somewhat working through the AX11000, but we completely lost access to the TP-Link web interface (connected from the AX10000 by Ethernet directly to my laptop), so I could not check any speeds, make any configuration changes, or even see what was still connected.

I finally gave up on getting the IP Passthrough to work cleanly and have decided to cut my losses in setup and equipment (hundreds of dollars of TP-Link devices) and migrate everything over to the AT&T system.  The AT&T fiber gateway/router only has 4 Ethernet ports, so I may have to add a switch to get everything on my home network working again, but without a solution from TP-Link directly I think I'm stuck.  I was told by several people at AT&T, before buying the fiber internet, that the IP Passthrough would work with my router, but I have nothing in writing and I don't want to give up the AT&T fiber 2.5 Gbps download and upload speeds.  Live and learn .