WebSockets not working when application is built - asp.net-core

Question

I have got to ASP.NET-Core 2.0 apps communicating via WebSockets.
App A is Server.
Application A is running on a remote server with Ubuntu.
App B is Client
Application B is running on a PC setup in my office.
When I test my applications locally in Debug everything works fine. Client connects to the server and they can exchange information.
However, when I build my Server app, Client can connect to it but when server tries to send a message to the client the message is not received by the client.
public async Task<RecievedResult> RecieveAsync(CancellationToken cancellationToken)
{
RecievedResult endResult;
var buffer = new byte[Connection.ReceiveChunkSize];
WebSocketReceiveResult result;
MemoryStream memoryStream = new MemoryStream();
do
{
if (cancellationToken.IsCancellationRequested)
{
throw new TaskCanceledException();
}
Console.WriteLine("Server Invoke");
// result never finishes when application is build. On debug it finishes and method returns the correct result
result = await _webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), cancellationToken);
if (result.MessageType == WebSocketMessageType.Close)
{
await CloseAsync(cancellationToken);
endResult = new RecievedResult(null, true);
return endResult;
}
memoryStream.Write(buffer, 0, result.Count);
} while (!result.EndOfMessage);
endResult = new RecievedResult(memoryStream, false);
return endResult;
}
This is the part of code where everything hangs.
What I tried was:
Build Server - Build Client => not working
Build Server - Debug Client => not working
Debug Server - Debug Client => working
I need any advice what might be wrong here and where I should look for issues.
Console if free of errors. Everything hangs on:
result = await _webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), cancellationToken);