I’d like to add one recent experience - it may be something in your immediate setup or a software problem, but it doesn’t have to be.
My ARC was working fine. Then one day it stopped working, can’t connect to core. A port scan from my phone suddenly showed the ARC port as closed. The router still had the port forwarding in place and previously the port had showed as open in the scan, as it should. The Roon ARC settings showed “Ready”. It was close to midnight, but I could not bear it and started taking things apart. Hours later I gave up and went to sleep, way too late.
In the morning I dragged my carcass to work. Lo and behold, it worked from the work’s LAN. I installed ARC on a colleague’s phone, using a different mobile provider, it worked. My phone still did not.
Driving home in the evening, I started preparing for a terrible night of stabbing in the dark again. Coming home, I tried again and … it worked from my phone. Then I learned that, starting the previous day, there had been some kind of large-scale cell phone outage in the German cell network, with some providers not being able to establish service at all, while some, including mine, not affected much. I don’t know for sure, but it’s possible that when this started, they might have shut down non-essential services quickly, and, who knows, may have closed down ports within their cell network.
Lesson learned: Patience is a virtue. 