I guess we will agree to disagree. Anything that is written to operate over a network should be able to span networks easily. In any event, it needs better documentation about this limitation.This isn't something Plex needs to fix. It is not intended to broadcast across subnets.
The plugin has a setting "Disables security. May be needed for initial access." in its configuration menu (FreeNAS WebUI > left panel Plugins > Plex).
Did you try changing changing that?
I think this is needed when accessing plex from a different broadcast domain, before setting a password.
That might have been part of it. I think I tried both ways and still got the same results.