I’ve achieved remote access to my Roon core running on my Synology by running a VPN on the NAS and connecting to it on my iPhone. I haven’t tested performance yet but I imagine that’s the major concern with implementing and supporting such a feature.
I’ve worked around this by installing Softether VPN server on my Windows pc. I’m then able to connect my iPhone to the VPN while away from home, and use Roon as though I were home. I do agree though, this capability should be native in Roon.