I don’t know if this will be useful to anyone but I have had the same problem with Nucleus not getting an IP address. I did manage to get it working again, could be a fluke.
I connected a monitor to the HDMI A port. All I got was a blank screen with ‘roon’ written across the centre of the screen. I was going to return the Nucleus to my retailer. By luck I happened to disconnect the ethernet connection first. I had a screen message ‘searching for network’. I reconnected the ethernet cable. New message reporting IP address Mac address and an indication that the web interface ws now available at the quoted IP address. My remotes could now access the roon core. Normal operation resumed.
Hope this helps somebody