Roon does work when the internet is down except when it needs to to verify the subscription.
What remote device are you using?
IPAD’s don’t work with Roon when the internet is down. Don’t know why but I’ve experienced that myself. My android phone and windows laptop did work with the internet down.
It’s variable, some people’s systems do work without an Internet connection and some don’t. I personally think it is down to the characteristics of the individual network equipment.
It should work offline but…