I have some problems with my new Plex-server. I have the server installed on a notebook, which is connected via wire with my internet. On my PC, which is connected also via wire, I can only watch with ultra low quality when I use the web-client. With the dedicated Windows-Client, I can watch on highest quality.
The problem now is: I shared my media-center with a friend, which can also only watch on super low quality when she uses the web-client AND also on the dedicated Mac-Client. Her internet-connection is really fast and she shouldnt have any problems but she is just not able to set higher quality than ~360p. In the apps, the quality-setting is set to the maximum.
in the server settings @ https://app.plex.tv/desktop > settings > remote access > make sure your upload speed is set correctly, and make sure limit remote stream is set to original (plex will automatically transcode down to meet your upload speed and number of clients)
in each client settings, there is a quality preference that needs adjusted, I usually put them all on original quality but you can set them lower (for example a phone with a limited data plan) for less data usage.