I don't believe this is true, do you have more details?
It's needed for authentication but locally, it should be nearly fully functional. At worst, you should need to set up a subnet that's allowed without auth in the server's network settings.
Aside from that, internet is mostly needed for downloading metadata and the server can work just fine without an internet connection.
While you're experimenting, try to set up the authless subnet in your settings. Settings -> Network -> "List of IP addresses and networks that are allowed without auth".
It's needed for authentication but locally, it should be nearly fully functional. At worst, you should need to set up a subnet that's allowed without auth in the server's network settings.
Aside from that, internet is mostly needed for downloading metadata and the server can work just fine without an internet connection.