You should be able to find plexamp logs, zip and drop them into your reply for Elan.
At a terminal, maybe run this command and paste the output into a code block </> in your reply in order to verify that TLS connections, certs, and name resolution are okay.
curl -v https://plex.tv
When I roll a new 64bit raspbian lite, I sudo apt update, sudo apt full-upgrade, reboot, set locale, TZ, hostname, WiFi country, static IP, DNS servers 1.1.1.1 and 9.9.9.9, hostname, reboot, node, and plexamp.
just saw your edit how you did a bunch of those 
