Update. I finally gave up on D-Link's o/s and installed the DD-WRT version. Wish I had done it years ago. I now have access to all settings, even TX power settings, and no bugs detected yet. Wired Ethernet speeds are around 930 Mbps and WiFi to our phones broke the 290 Mbps barrier-- 400+ Mbps now, even two rooms away.
And, my wife and I work from home, totally dependent on the network. For the first time I had no disruptions for a whole day using WiFi calling to the cellphone provider, and no disruptions using Cisco WebEx either.
To load the DD-WRT o/s, I first had to wipe it clean of D-Link's version 1.21 and go back to 1.12 (which I still had as a .bin file on my PC), as outlined in another thread here. Thanks for those instructions Fuzzy.