Check the bandwidth settings on the server and on the client.
Plex default client settings for remote streaming are 2 Mbps / 720p.
Server:
Settings → Remote Access + Show Advanced.
Verify both Internet Upload Speed and Limit Remote Stream Bitrate are correct.
Client:
Android Mobile is Settings → Quality → Remote Streaming Quality. Others should be similar.
Note: Enabling Automatically adjust quality (if available on the client) results in Plex Media Server transcoding the stream. See https://support.plex.tv/articles/115007570148-automatically-adjust-quality-when-streaming/.
