Just a note for anyone else searching for UDM Pro No Internet Detected issue.
I have a Netgear cable modem so it is not a router at all. I installed the UDM Pro but it could never get to the Internet. I searched and found some references to setting the time but my time was correct. I then read something about the UDM Pro wanting to get to “something” but not telling us what. For some reason, that clicked as a potential DNS issue. Sure enough, on the Advanced options on the UDM Pro, I saw the Internet set to DHCP as it should be but no DNS filled in. Since this is a router, it appears that the DHCP from Spectrum did NOT send the DNS servers along. I filled in DNS with the 184.108.40.206 for Google and it worked.
Note the UDM Pro updated itself but never left the screen. After 10 minutes, I went to the default 192.168.1.1 again and it came up with the full interface.
I hope this helps someone else.