You on any mesh network at all. Can you confirm if the iPhone is on the same network range as your core? If so perhaps removing and reinstalling the phone app may help.
Have you tried forcing it connect, click on help and then enter in 255.255.255.0 and see if it finds it.
anyway… tried again right now and… it did find the Core
thank you
btw… yes: I did check the iPhone, when the snafu happened, was on the same network. tried controlling other network connected stuff (Philips Hue lights, Netatmo thermostat) and… it was. just Roon was affected