Your TIDAL subscription is on the Intro plan. That plan only provides thirty second previews when TIDAL is used through another application such as Roon. That is why tracks play only part of the music and then stop. It is not a fault with your Nucleus One or your network.
To play full tracks in Roon, the TIDAL account needs to be on one of their paid plans rather than Intro. Once you upgrade, playback will work immediately with no changes needed on the Roon side.
On the other part, we are not sure what you mean and would rather ask than guess. When you say you can only download half of your playlists, do you mean:
That your TIDAL playlists appear in Roon but only some of the tracks inside them are usable
Or that you are downloading music onto your phone or tablet with Roon ARC, to listen offline
Please also tell us: have you added your TIDAL albums and playlists to your Roon library? At the moment your Roon library contains only one album and one track in total, so almost nothing has been added yet. That may simply be because the account is new, but it would explain a lot of what you are seeing.
Once we know which of the two you meant, we can give you a clear answer on that as well.