@RonTheRed   Follow-up. I have a work around.

The 3.11.6.1 firmware for the Giga Hub rolled out, evidently, from 1 May to 5 May. I went to bed on the night of 4 May, and in the morning of 5 May, PPPoE was no longer working with my TP-Link AX20(CA). There is talk that the update might have broken PPPoE. Finally, I tried to connect using PPPoE on my laptop connected via Ethernet cable directly to the Giga Hub, and THAT did not work either. This means that the PPPoE problem is in the Giga Hub, and not the AX20. In the meantime, I even went out and bought a new router, because I was getting desperate!

So, the next thing to try was the DMZ. I tried this several times and had no luck. Then I chanced across a post  here:   https://forum.bell.ca/t5/internet/using-pppoe-and-dmz-advanced-dmz-for-bridge-mode-use-of-3rd/m-p/705

I set my Giga Hub to DMZ ON and checked the ADMZ box, then entered the MAC address of the WAN port of my router. I then went to my router, and this is where I had made errors earlier. I had set up the Giga Hub for ADMZ operation, but never changed the settings on the router! So, the link above told me to set the router to acquire the IP Dynamically, and to CLEAR THE "b1" CREDENTIALS from the PPPoE text boxes.  Boom! I was in.

I hope that this post is of service to someone who has something better to do with 15 man-hours of their time. Have a good day, all.

PS  My new router is a BE400, and it is a beast! My link speeds went from 150MBPS or so, up to over 400 on a 500MBPS link! Not taking the router back!!