You’ve got a really complicated setup. I would suggest simplifying things and making sure PMS is working properly before making it more complex. That will also help identify where the problem is.
You basically have 2 networks going on with a double NAT.
and
The working iPad is likely using your server’s public IP address or Plex’s relay connection depending on if your remote access is working or not.
The non-working ones are likely trying to use the secured local plex.direct address and failing which most likely points to possibly DNS rebinding protection being enabled on the firewall. Or does your firewall have IP isolation capabilities? That can also prevent devices from seeing each other.
