Sure seems like you may be experiencing duplicate IP addresses in your router. 

First, do not do any factory resets, it will just cloud the issue.

I’d recommend that you unplug all Sonos devices from power. While all of them are unplugged, reboot your router. Give it a couple of minutes to come back up, then plug all your Sonos devices back in to power. They’ll ask your refreshed router for ‘new’ IP addresses, and get them.

Longer term, look at your router’s manual on how to assign reserved IP addresses, and do so, at the least for all your Sonos, and preferably most of your network devices…but I’ll leave that to you. It’s just good network etiquette. 

Thanks @Airgetlam.  After reading up on in a bit more it does seem like that is the most likely issue.  Will try it later and update whether or not that worked.  

It is so easy to try that in situations like yours setting static/reserved IP addresses is the “go to” first choice.

If it isn’t the issue, no harm done and it may prevent future issues.

I reset my router while all the Sonos equipment (2 Amps, Arc and Sub) was unplugged.  I plugged the Arc back in first.  It still works with the TV its attached to, but I’m still not able to add it.  

At this point the app is initially recognizing it and asks me if I want to add it, but when I try to do that I get an error that “Your mobile device couldn’t connect to your Sonos Arc.  Make sure the Sonos Arc is connect to power”.  It is connected.  And then there is nowhere to go from there.  

Not sure if it matters, but the connection to the TV is via HDMI/eArc and then I have the ethernet directly into the TV and Arc.  I do believe my electrician split one ethernet cable into 2 (1 for TV and 1 for Arc), because he only ran one to the area initially.  Maybe that is causing an issue?

That does make me wonder if somehow the ‘radio/wifi’ was turned off on the Arc, it seems as though it’s having issues properly communicating with the rest of the system. Perhaps somehow your electrician turned it off, not understanding the rather more complex nature of communication between Sonos devices?

I’d also probably recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.

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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.

And now I just randomly tried to add the Arc again, and was able to without making any changes.  The issue now is that the app doesn’t recognize my Sub, which it did before.  I think it was “lost” when I reset everything, and now I can’t even get to the point where I can initially add it.  I’ve unplugged it and plugged it back in several times.  I’m hesitant to reset the router again and lose the Arc, but not sure what to do.

Update, I got on the phone w/ Sonos and got everything squared away.  I’m not quite sure how, since he didn’t ask me to do anything I hadn’t already tried, but it worked and I’m fine w/ it...for now.  

The one thing he did have me do that I hadn't tried since first trying to figure it out myself was to factory reset each of the components after resetting the router.  I read here not to do that.  I brought it up w/ him on the phone, and he didn’t agree.  I went with what he suggested and it worked.  Was basically to reset the router, factory reset each item and it worked.  

We discourage a blind Factory Reset because it usually causes mountains of extra, unnecessary work. Sometimes a Factory Reset is productive. If SONOS phone support is supervising the Factory Reset, it is probably warranted.