.NET 5 Server App on GameLift?

The GameLift C# Server SDK does not officially support creating C# server builds outside of Unity. However, the SDK is essentially a utility for sending websocket messages to the websocket server on the EC2 instance, so if push comes to shove you could write your own C# .Net5 server SDK. Folks have done it in the past for other languages, see:

1. Rust: https://github.com/ZaMaZaN4iK/aws-gamelift-server-sdk-rs/tree/main/src
2. Go: https://github.com/neguse/gomelift
3. Other community projects: https://github.com/favoyang/awesome-gamelift

However, before diving into that, it might prudent to find out why your server process crashed. First, it's best to test out your server process with GameLift Local first. Testing locally will save you the trouble of having to wait for fleet creation. See: https://docs.aws.amazon.com/gamelift/latest/developerguide/integration-testing-local.html

Next, it might be worth remoting to the fleet instance and run the server process directly on the instance to see if you see any helpful error message. Make sure you create your fleet with RDP port (3389) opened. See: https://docs.aws.amazon.com/gamelift/latest/developerguide/fleets-remote-access.html