And, yet, here is a screenshot of my iPhone with WiFi turned off (cellular data only) playing an episode of “Around the World in 80 Days” on the iOS Plex app while signed in to my server.
The machine running the Plex Server has a static LAN IP address. My ISP gives me a dynamic WAN address. Sometimes it lasts a day or two. Sometimes it changes several times in a day.
I have port forwarding set up for port 32400. So, yes, it is open.
Quite often, but not always, I see this on the server: