I have a new Windows 2019 Server box in the lab, and when I promoted it to a Domain Controller, after the promotion the network adapter was reporting "not connected to the Internet", even though clearly the box was connected to the Internet. I found this Microsoft TechNet post that had the answer for me:
Server 2019 "not connected to the internet" issue after domain join or dc promotion (microsoft.com)
I went into Settings > Network & Internet and found a link that says "Change connection properties".
After clicking that link, suddenly my error message went away. See the post link above for details.