Well, after plugging the Nucleus+ directly into my router, rebooting and swapping out cables, I was finally able to get Roon up and running. It appears that the ethernet cable, which had worked perfectly with my Innuos server, was the problem after all.
I had wondered if there was also an issue with the powerline adapter but plugging the Nucleus+ back into the adapter, it connected seamlessly.
Thanks again for your posts and suggestions. I found them very helpful, as it made me realise that the Nucleus+ wasn’t talking to the DHCP server, even though it was visible on the Roon app.