Unfortunately, support doesn’t live in these community forums, you have to call Sonos Support directly to discuss that diagnostic.
There are two possibilities that I can think of, since I don’t have access to diagnostic data. First would be the ‘radio/wifi’ has been turned off on the Arc for some reason. Because the Sub connects via a hidden (to us) 5Ghz WiFi signal, it needs to be on. On both the Sub and the Arc, now that I think of it….but you said the Sub connects to other Sonos devices, so it’s likely the Arc. And if it ‘appears’ to be on, I’d turn it off, wait a minute, then turn it back on, I’m paranoid.
The other possibility, that no one in this community would be able to see, is a hardware failure, something I would think might show up in your diagnostic. Resolving that issue would require direct Sonos intervention anyway.