Additionally to the server remote connection settings, the iOS Plex app has its own bandwidth preference for remote connections. Verify that it is not limited to 2 mbps.
Another thing to check: is your server directly(!) accessible from the outside?
If not, Plex is reverting to the Plex relay connection – which happens to be limited to 2mbps for Plex Pass holders.
(1 mbps without Plex Pass)
