My iPhone app is stuck in the “Looking for Roon Core” state for 24 hours now. It just suddenly stopped working after being fine for years. When I tried to connect it would cause the Roon application on my iMac which is running the Core to quit. Restarted computer, phone and router/wifi network. Hit the Help button and did the IP address thing. I can still connect via a laptop though.
Running the Core on an iMac (Monterey 12.2.1). I have two systems. Both use Bluesound devices that connect to wifi via ethernet cable to routers/extenders. All software is up to date. Phone runs IOS 15.5.
Check which WiFi network you are actually in with your iPhone. I once got accidentally into my guest network - well quite frustrating until you notice.