The explanation you received/understood doesn’t match my understanding of the system…but then access points are not in my area of knowledge, either.

The understanding I have is that the Sub receives a ‘proxy’ IP address, through the network connection to the Arc. The Sub communicates to the Arc across a ‘hidden’ WiFi connection on a 5Ghz channel. This is why you shouldn’t ever turn off the radio on any Sonos soundbar, it defeats that connection. They can, I believe, go back to the router if both devices are wired to the router, but not wired to each other. I consider this less than advisable, as there may be a slight delay due to extra routing. I suspect my concerns are unfounded, but I’ve never tested to check, and it would ultimately depend on the router’s speed, and not Sonos’. 

I suspect your issue is with the AP not being set up properly. Or maybe the ‘WiFi/radio’ got accidentally turned off on your Arc, it’s hard to tell. But I agree that ‘SonosNet’ is not an option for you, as you point out.  The newer devices don’t work with SonosNet.

I haven’t turned off the radio on the arc? is that a setting??  I attempted to reboot and re-add the arc. I then readded the sub. The sub is listed under the arc in the arc settings, but in the main list of devices the sub is listed by itself, as not connected? When I play music through the arc I get no music/ sound from the sub? 
 

Yes, turning off the radio on the Arc is a setting, on every Sonos device except maybe those that have no Ethernet ports. 
 

What is ‘the main list of devices’? Can you post screenshots, please? 

Here you go

Thanks for that. 

I don’t understand it at all, to be honest. It does show as being disconnected on the first screenshot, yet connected on the second. 

I’d likely try a few things. First, I’d reboot my router. Always a safe thing with dealing with possible network shenanigans. Then I’d power cycle both the Sub and the Arc, to see if that makes a difference. If it didn’t, I’d use that ‘Remove Sub’ button, and go through the process, and then add it back in, to see if that made any difference. 

Finally, if none of those steps satisfied the issue, I would submit a system diagnostic, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Yes, it is confusing and it’s frustrating. I cannot simply add the Sub to the living room. It adds it then ‘falls off’ immediately and shows the ‘sub not connected’ as above. I’ve tried re-adding it repeatedly and power cycling both the ARC and the SUB. It’s really poorly designed as they are phasing out SonosNet. Thus the only way to get it to work is to plug in an old gen1 even though the ARC and the Sub should connect wirelessly. The unfortunate piece is I don’t have a wired connection that I can access. 

So basically I have a Sub that will not connect to the ARC and hold the connection (even though they are right next to each other) and that is what they supposed to do.  As I said, it does connect then immediately falls off. 

SonosNet isn’t involved. One way or the other, it just wouldn’t make any difference.

What did Sonos Support say, when you called in?

Correct it is not. BUT if I have an old Play 1 plugged in with Ethernet the Sub works using the Arc. It will not work with the Arc with WiFi. Sonos tech said to enable multiple broadcasting on my access points which raises security concerns when changed.