Tell us what router you have. In the meantime, disabling 5GHz on that router will be the quickest fix. Do not factory reset anything, its just a waste of time for router issues like this.

@controlav  I have a unifi dream machine, nothing is being blocked on the way out.

I don’t believe anyone said something was being blocked by your router so I’m confused by your response above.

I’d try the suggestion as a first step.

I don’t usually use Apple so my situation is a bit different. What I found I needed to do to get my system stable was to assign static/reserved IP v4 addresses to my Sonos (v6 hasn’t been an issue) followed by shutting all Sonos down and rebooting the router, then powering up my Sonos.

I spent a lot of hours resisting doing this and digging through my router’s logs looking for a DHCP (v4) issue but finding nothing. No duplicates, no unanticipated changes, no errors, nothing out of the ordinary expected activity.

Since the change I have had no issues with updates or power failures. I just wish I could prove why.

I hope you have configured that UDM correctly for proxying SSDP and the other critical things that these complex devices require for Sonos compatibility.

@Stanley_4  Thank you for your assistance, the statement about nothing being blocked was just extra info about the system.

I have tried disabling 5ghz and am yielding the same results. I will try your suggestion of assigning static IP’s.

@controlav Thank you for the assistance. If you could be more specific as to what the needs are. Like a list of items, im at a loss for what your pointing to. Is there  a config list or whitelist needed listed somewhere? I know it has worked for over a year with no issues at all. Then the Sonos app updated and it doesnt link or show in the app.

In further troubleshooting I replaced my current system with a full Eero setup. I had the same issue, I then installed my Firewalla gold and put in my zyxel access points. This also yielded the same result.

Can we rule out an issue with the iphone and app? Maybe related to Iphone hidden IP feaure or something? (I havnt played with this, but maybe I should)

I was able to factory reset all the device on the 4th attempt and get them setup. They did disappear from the app about 15 minutes later, but at least the system is functioning correctly. The surround works and as long as you dont log into the app you would have no idea there is an error.

I have no idea how to set up UDM, I use Amplifi devices to get the advantages of great Ubiquiti network hardware without the insane configuration required of their Pro level gear. “Ain’t no-one got time for that”. I have read enough on this forum and Reddit to know its not plug-and-play.

@controlav I understand its not for everyone and appreciate your help. I do have my N+ cert and do install networks for a living. I have a pretty good idea of the basic setup needs for a UDM and other routers, I dont consider myself a unifi master by any stretch as I dont specialize in it and theres a reason I am running at my home, but I have a decent general working idea of the system. I have also tried multiple systems at this point and yield the same result, which tells me its not a network issue.

As much as I enjoy the Sonos ecosystem, perhaps its time to move on.

I finally have this solved and my SONOS system is now working as intended and better than it has in a long time.

This was not a network issue on my end, but a preference on how the SONOS system prefers to operate. This issue only truly shows up when you have speakers grouped like a surround system.

The solution: Hard wire any one of your SONOS devices to your network. This will remove all SONOS devices from your WiFi and allow them to setup the SonosNET network. This is the preferred network for the devices to connect and you will have no issues.

So it was a network issue, which you fixed by using SonosNet instead of your WiFi, which is a solid solution to bad local networks. However as your phone will still use your WiFi, you still have the potential risk of your local network messing up the ability of comms between the devices on SonosNet and your phone. You’ll know as they will all vanish, not just one or two. Good luck.

@controlav I appreciate you were willing to assist and thank you for trying.

If it makes you wish to blame my network and not the product, please do so. I dont see this as a network issue, but a SONOS product issue. I plugged the device in instead of using the WiFi and it setup and configured the SONOSNet network which was missing. This was an installation error and my fault. I accept that. This is also a failure in documentation from SONOS. This one step solved the app not connecting the grouped speakers correctly. Why not just say in the documentation that if you are grouping devices, please ensure one SONOS device is hard wired, so that the SONOSNet can be configured for your system?

I reached out to some colleagues that install SONOS for a living. They are the ones who shined some light on this issue and how to fix it. It all stems from how SONOS tries to switch between using your wifi and the SONOSNet system and gets confused. A simple cord fixes all this.

Again, thank you for your help. I wish you all the best.

Cotrolav is right. Changing from connecting to the WiFi network to the Ethernet and using SONOSnet is a network issue and not just a documentation issue.  
 

Bottom line, the SONOS are wireless speakers designed to be grouped together on a WiFi network and they do not retain the network setting necessary for to keep functioning. Having to plug them in with Ethernet is not the solution. 
 

I have the same problem when grouping speakers together on a WiFi network. Eventually the speakers are not connected and I have to go through the whole process to connect them. This is a more recent issue, I have had the same speakers for 7 years and rarely if ever had to do that previously. I will try blocking 5g. 

I am curious if that works for you. I know for me it didn't. The only solution that worked for me was to simply connect one to ethernet to one speaker in the sonos metwork. Since doing it, I have not had to regroup a single speaker. They just work and it's worked flawlessly for months. 
I did try to unplug the ethernet just the other day and surprise!! Issues with grouping and finding speakers. Plugged it in and all worked immediately. 
I have an ethernet outlet next to one speaker, so it isn't am issue for me and it fixes the issue entirely for me.