"It works for me" is not statistically significant.

The important things for people reading to keep in mind:

1) This user has provided no context for it 'working' for them. If their use case to the United States is streaming, Plex defaults to 720p HD 4mbps. That is far, far below the server's limit of 1000Mbit and easy for many servers to reach.

2) The original poster is looking at using a Hetzner server with Amazon Cloud Drive. If you try to upload to Amazon Cloud Drive (US) from outside the US, you may find speeds significantly slower and inconsistent. That is what one would see with a Hetzner server. Hetzner (DE) to Amazon Cloud Drive (UK) might work well. Hetzner (DE) to Amazon Cloud Drive (US) would be sketchy on its best day.

3) The usual method for linking Amazon Cloud Drive to Plex is acd_cli, encfs, and FUSE. That isn't particularly stable itself vulnerable to Amazon Cloud Drive oddities, network issues, and similar glitches even on native US networks. When you add international bandwidth to that or providers that don't peer well like Hetzner, you are adding in trouble.

4) The issue with peering isn't something many care to try to understand. If the server is up on the network, streaming at 4mbps, and playing on the Roku, it works. How can I say it doesn't work? It works. See it working?!

...except... The 1000Mbit port is significantly slower from the United States even though you have 200Mbit or more upstream. Why? Well, it works. It just works at 50Mbit or less because of peering. You'll find the Hetzner server downloading at 60 megabites per second from one location and struggling to get 1 megabyte from somewhere else. That is also due to peering.

Better providers with better peering will allow you to reach closer to your port's maximum even from the United States. That isn't due to congestion on Hetzner's network. Remember, you can get 60 megabytes per second testing from within Germany or Western EU. It just slows down significantly from New York or farther into the US.