I have been encountering some issues as of late with the streaming resolutions on plex. For some reason in the last week or so I cannot play any movie/video over 720p resolution even though maximum resolution is 1080p+ The plex server doesn’t even give the option to play in 1080p etc, instead throttling everything to 720p.
Are you streaming from within your home network or remotely?
Sounds like your remote access might have gone bad, causing the connection to be routed indirectly (which will limit the streaming bandwidth to 2 Mbps for Plex Pass members, 1 Mbps for regular users).
Edit:
You can confirm this by double-checking the server’s dashboard / the “now playing” tile which will show the type of connection and if/how video/audio/subtitles are being transcoded.