Audioman: if you are referring to your internet service, its going down should not keep you from accessing local files. What you are seeing is that Roon stops working when the Internet is down. What you need is a Roon alternative that plays local files from your network.
Foorbar2000 is one of the simplest. J. River also well known. Plex the most sophisticated from a network perspective. The all take configuration and knowing basic network - like how to find a NAS over wifi or ethernet.
If your fanless computer is Windows or Mac you'll be able to set something up that allows you to have a similar local file experience to Roon. If you're using Rock or have a Nucleus (Roon OS computer) then you won't be able to use it for much else and you'll need another computing device of some sort. Many streamers will support local files.
What you want to do is pretty basic to a digital audio veteran. But yeah it's a new world from analog or if Roon was plug n play for you. Try Plex...