@Mark_Allison, welcome to the Community. Two quick questions:
What version (the Build number) of Roon are you running for your Core on your iMac? And what version of Roon Remote are you running on your iPhone 7? Both versions need to be the same for the Roon Core to connect to the Roon Remote
Is your iPhone running on WiFi, and if so, can you confirm that it is connected to the same network as your iMac Core?
Have you tried rebooting your Core and iPhone? When you say ARC works, is that over WiFi at home or outside the home over a mobile connection? And can you confirm your ARC connection is being made to the correct Core?
Do you have a VPN or extra security applications running on the iPhones? Can’t say I have had issues with connecting with my iOS devices unless my Windows Core was set to Public vs. Private network mode (Private mode allows discovery on the network). I have not seen that option on my Mac Core.