A UDP Server can't send response back to sender via router

For testing purposes I have written a UDP Client and a UDP Server app listening on a specific free port. This works well on a local LAN between computers but not when I put the UDP Server app on a remote server. The client end is behind a residential router. I send text from the client to the server, and the server receives this text and tries to send a response back to the sending IP/Port, but that produces a Socket Error 10054, Connection reset by peer on the UDP Server app. The sender port in the packet has been modified by the router to a random number, as it should, and that is where the

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