That being said, I would run some other tests, to other endpoints and with other speed test providers, to confirm or deny what’s going on. Then, if I could nail down a pattern (e.g., routes to Madtown suffer but routes to Portland ME are low latency), I’d run it up my ISPs flagpole as a service issue.
Upload speeds, on many internet connections, have been known to be capped -- typically a provider-enforced cap. What type of connection are you using remotely? Fiber, cable, hotspot, etc.?
I don't use Plex, what kind of bandwidth does it need? If he has good download you could set up a small cloud machine to proxy the traffic... maybe... I mean I don't know anything about Plex but in theory this could work
I decent 1080p video file can be around 3 Mbps. I thought my setup would be fine since I pay for 30 Mbps upload speeds, but it seems I don’t actually achieve that outside of my local area. I would fear that a cloud machine would suffer the same rate limit