I think I have a similar problem. I updated the version on my iPad first and when it tried to connect to the Nucleus it promtped me to update the Roon Nucleus to 1.8. I initiated the update and everything proceeded normally (download new version, started updating, etc.) and then it just got stuck. My two remotes (iPhoneX and iPad) are running version 1.8 and just get stuck waiting for the Roon Core (Remote Connection Waiting for Remote Core…)
I have rebooted my network and restarted the Nucleus a couple of times to no effect.
That DNS of 75.75.75.75 has been known to be unreliable and has caused issues with other Roon systems, can you try to change your router or Nucleus’ DNS to Cloudflare DNS, Quad9 or Google DNS and then try an Operating System Reinstall on the Web Interface?