Did you ever find a fix for this? I’m having the same issue with a similar setup (server on a different VLAN to remote, but firewall rule allowing specific access)
I don’t believe it’s HTTPS related as I don’t have strict TLS enabled and I can query my Plex server over HTTP. The only thing I wondered was looking at the session details, the Plex server is actually seeing it as a WAN connection due to the differing subnet. This is from the session xml
In case anyone else comes across this, the issue is indeed that Plex sees a client with an IP in a different subnet as a WAN client, and the integration then ignores it. As I’m running my Plex server on my NAS, I added an interface on my media VLAN and set Plex to specifically use this interface in the settings. Signed out the Plex clients (they were still reporting as WAN beforehand) and signed back in and then they correctly showed as LAN clients and the integration found them.