It appears your remote access setup is borked, causing your clients to use an indirect connection. Those are limited to 1 Mbps (2 Mbps for Plex Pass members).
The remote access status indicator will always briefly show as „green“ for a moment while Plex is checking the connection status. So seeing it briefly as „connected“ doesn’t mean it’s actually working.
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
