Safari can't log into Apple websites (discussion forums, developer forums, itunes connect) High Sierra

For a while now from Safari I have not been able to log into Apple websites that require a login. These forums for example, or iTunes Connect, or developer.apple.com. It says that Safari cannot open the page because it can't establish a secure connection to the server "idmsa.apple.com".

I've spent three hours on the phone with Apple Support over this. Ran through every suggestion in their books, even reinstalled the OS. Nothing fixes it. They basically said the only solution was to update the OS to Mojave. Which might not fix it, but it might.

So really, High Sierra doesn't work with Apple's own sites? That cannot be true. There would be some mention of it online, I would think? I can't upgrade to Catalina. I might do Mojave, but at this point I just want to leave this computer running High Sierra to maintain the highest software compatibility. I plan to get a new computer soon, so this one is a legacy computer.

I can use Firefox to access this developer forum and the other sites, but I find it super ironic that Apple themselves cannot help me to access their own sites from their own computer using their own OS which is only 3 years old.

Anyone have any suggestions on how to fix this problem without updating the OS?

Anyone have High Sierra and able to access these forums, iTunes Connect, or developer.apple.com?

Thanks!