Can you see the Nucleus on the local network? Does it appear in the device list that you may be able to see in your Router admin pages?
Can you see the Nucleus Web admin page by going to http://nucleus or entering the ip address of your Nucleus in your web browser?
Note for other models of Nucleus (plus, one, titan), the url for the nucleus can be found at:
If you can not see the Nucleus on the network, can you connect a monitor or TV to the HDMI port? You should see some diagnostic text. If everything is working, this will include the ip address of the nucleus. If no text is shown, then the Nucleus may be faulty. If the text is shown, but no ip address is shown, then there may be a network issue. The text may give you a clue as to what this problem is.
In your network setup description you mention ‘repeater’.
This gives rise to a couple of other questions:
Is Wifi involved in the connection from the Roon Nucleus to the Internet Router? It is recommended to avoid the use of WiFi to connect your Nucleus (or indeed any Roon Server).
Is the Nucleus on the same subnet as the computer from which you are trying to connect? If they are both on the same subnet, they should have the same first 3 parts of the 4 part ip 4 address (e.g. 192.168.0.x) assuming that your netmask (set in the router) is 255.255.255.0.