@Axel_Lesch @Suedkiez @Martin_Webster
I got ARC working! Well, using a workaround. I was reading some other posts on here regarding ARC issues and someone mentioned Tailscale and how it worked with minimum fuss for them. So I downloaded the Tailscale app onto the MacMini, created an account and logged in. I then downloaded the Tailscale app on my iPhone and logged in. It then said that I needed to turn on VPN on the iPhone which I did. I then opened ARC on the phone and it connected! All this with absolutely no configuration on my end.
I turned off WiFi and took the dog for a walk and for 40 minutes ARC worked perfectly on 4G. I tested using local files and I streamed an album on Qobuz that Roon recommended for me (Hysteria by Indigo Sparke) and it all worked perfectly.
The odd thing is that in Roon Settings > Roon ARC it is still saying that it cannot connect to the core and will not work, yet I can 100% confirm that it does work - i just need to turn on VPN on my phone when I go out.
Thank you all very much for your inputs - i got there in the end! 