Playlists to iPhone

Is there a way to export playlists to listen off line.

You’ve got to use Tidal’s native app for that use case, since Roon can’t download and store streaming media.
All functionality is hosted from the core in your home network anyways, so mobiles tablets and PC/Mac remotes won’t work without being connected to it.
I strongly encourage you to check out the knowledge base to better understand Roon’s architecture and possibilities.