Plex has a quiet plan B called Relay. When your server can't be reached from the internet directly, Plex bounces the stream through its own servers so playback still works. Nobody gets an error. Everybody gets 720p.
Relay only appears when direct remote access is broken, so the fix is to repair the direct path. Start with turning on Remote Access. If that ends in a red X, continue to port forwarding.
Eyeball spots this automatically, by the way. A watched server that slips into Relay gets flagged as limited quality on your dashboard, so you find out before movie night does.
I watch your Plex from the internet and send word the moment it breaks. The pay is nothing. The setup is one sign-in.