Looks like your phone updated but your NUC did not. Your phone on Roon 2.0 cannot talk to your NUC still on 1.8
I think the first thing you need to do is get your phone back on Roon 1.8 so it can talk to your NUC. The way to do that is go to the Apple App Store and install Roon Remote (Legacy).
You next choice is to try to get your NUC to upgrade from Roon 1.8 to Roon 2.0 or just leave it on Roon 1.8 until tomorrow when Roon @support gets back to work. I would wait.
Also, forget about the Roon ARC app for now. That is not going to work until you get your NUC upgraded to Roon 2.0.
Just remember, if you do get your NUC to upgrade to Roon 2.0, you will need to go back to the Apple App Store and install Roon Remote which is now on version 2.0. That is, unless in the process of upgrading your NUC to Roon 2.0, your iPhone also gets updated.
Can you reboot the Core and check to see if that helps? If not, can you please use the Reinstall button on the Web UI and see if this helps the Nucleus update? An OS reinstall should leave your Roon database intact.