Well, I called Comcast and they had me try the site https://downforeveryoneorjustme.com/ which confirmed the inability to load site https://www.tektonics.org/ was “just for me,” and then had me check http://test-ipv6.com/ and which I passed 10/10, and then they tried to change the modem security settings which did not help. I checked my History and saw that I was able to access that site in the last 7 days before i had this problem. And so they are going to send a new modem free (this one is at least 6 years old) and I can return the old one at my convenience. So thank God for that, but it will be about 3 days before the new one comes in and I can see if that fixes the problem.
I can access my modems settings myself if i could remember the user name and pword since I changed it.
Daniel, I think it has something to do with your software.
You could download a copy of a self-booting operating system environment like Knoppix and if that boots up and its browser gets to the site, this should prove a software issue with your Windows.
Knoppix download:
https://www.knopper.net/knoppix/index-en.html