Thanks for your reply and I have successfully reactivated the six sceneries.
As suggested, I did get the new QLM keys for all my FSDT scenery, since they were originally installed in P3D with the old Esellerate keys in Feb 2019.
Also, I checked, and my computer's firewall is not blocking Couatl.exe, couatl64_P3D.exe, or the QLMLicenseWizard.
Edit update: Just read the thread "Airports missing after update" and remembered something I did when attempting the reactivations. If I used the new QLM key, the QLM LicenseWizard appeared and the activation would not continue. So, assuming I had nothing to lose, I closed and reopened P3D, opened the Addon Manager, and entered the old eSellerate key (beginning with "FSDT"), and clicked "OK". To my great surprise, the scenery was activated. I later checked the *.reg file created in C:\Users and it contained the new QLM key.