Offizieller Kommentar
@Paul > You don't have a dud, you're just not doing it right!
To connect to rekordbox over ethernet, you need to ensure your system is configured to allow the traffic through the firewall. See this article for more information on troubleshooting that.
In terms of streaming, you can not capture input audio from the RX as it an output-only interface (sound card). You may try using the master output returned to the internal audio interface of your computer to achieve that.