Fire stick will only access over indirect connection after waking up from sleep

[FordGuy61]

See second half of this thread:

Only 0.7 Mbps Video Stream with 4 Mbps Upload Speed

Very likely, your server is not directly reachable from the outside. So the traffic is relayed over the Plex cloud servers. And this relay service is bandwidth-restricted (1mbps normal, 2 mbps with Plex Pass). see https://support.plex.tv/articles/216766168-accessing-a-server-through-relay/