Hi Connor.
Thanks for you reply.
Note that your reply was only just emailed to me today (16 Mar here), but is posted as 11 Mar on Roon - delay in notifications?
Problem resolved itself after a few days - just started to work again, but with no changes by me to either the server or to the phone. So no idea what caused the issue. Hopefully it will not happen again, but maybe to find out answers your questions:
No iOS devices registered - what does this mean?
Setup is: Internet to server, server to OSX all ethernet. iOS to server WiFi. All on the same network.
iOS could ping the server no issue, but no response.
iOS could access all other network connections no problems.
VPN always on for iOS and OSX, not on router/modem.