Evan , your original question was ;
As all of your listening seems to be from Roon, Mosaic is not a Roon control point ( though Rossini is a certified Roon end point). However I am not a Roon user and cannot offer any related advice. Maybe another member with relevant experience can pick up on this. I presume that doing this you use Roon Remote as the control point.
You did say, however,
So what were you listening to then? As I mentioned earlier for local file replay Mosaic requires that storage to interface with it via UPnP server software ( MinimServer is recommended by dCS).
NB : The configuration of network switches can affect whether Mosaic will discover Rossini and, if it does, the resulting operability. This may be an area to explore later if there is no obvious explanation. Did you check Rossini’s IP address again? You can find it via the Rossini front panel information menu/Unit Status.
As far as ethernet cables are concerned there are numerous archive posts here. I link an example. dCS recommend basic unshielded CAT certified cables e.g CAT 5e or unshielded CAT 6. However there are other users here that have Transparent.