I followed the suggestions in this discussion for diagnosing a client-side SSL error, but I don't know how to interpret the result.
The openssl command for www.plex.tv returns a good certificate. But when I try to login at https://www.plex.tv/sign-in/ Chrome strikes out the https and shows "Not Secure." I get an error message when I proceed to login. This happens on Firefox as well as Chrome, and it happens from any computer on my home network. I am able to login from an android phone over the cellular network.
FYI I ran the SSL command on a Synology NAS which also runs on my home network.
If it was an ISP problem, wouldn't the openssl command return a bad certificate?