The ROCK should startup even without a network connection at all. Question is what to do with it? Only things known are connecting a monitor to HDMI and a keyboard to USB to enter the command to reset the network configuration. If you start-up with a network connection but without internet access you can also access its configuration page. Roon Server should also start on the ROCK but without internet connection possibly can’t check the licensing status/authorization and thus won’t let remotes use Roon.
Note: If I understood Danny correctly, Roon may still work for some time without internet connection, there is just no longer a guarantee for that (and an internet connection may be mandatory for Roon to check licensing status/authorization on start-up for Roon to work normally but that may already have been the case before Roon 2.0).
NUC/Rock starts, but Roon will not work without internet.
I tested this and, With the ROCK/NUC running and functioning, I disconnect the ethernet cable to my network (disconnecting from internet). Roon kept working. Not sure how long it would take to stop functioning as that depends on the license check time limit.
If there is no internet and you restart (or start from cold) the ROCK/NUC, Roon does not function. YMMV