Interesting to know:
I switched my roon server now to run on my Synology NAS which has got a bit more power than the previous machine (MAC mini 2014 i5).
As this NAS is accessible by external through my VPN I wondered if roon is now able to find endpoints outside also (if in VPN). Big surprise! It works! I switched on VPN on my iPhone and started roon and immediately I was able to play from my library!