For starters… can you confirm your connection is listed as remote or indirect?
You should be able to see this on the server dashboard’s now playing section.
https://support.plex.tv/articles/200871837-status-and-dashboard/
With an indirect connection, the client cannot establish a direct connection but requires a Plex Relay server. Those connections are limited to 1 Mbps (2 Mbps for Plex Pass members).
This generally means there’s some issue with the remote access setup.
Did you already have a look at the related troubleshooting support article?
https://support.plex.tv/articles/200931138-troubleshooting-remote-access/
There’s also a guide with some more details on how to troubleshoot remote access issues:
