The Factory Reset complicated your issue. I suggest that you refrain from Factory Reset without further consult. If you have other SONOS units that have not been Factory Reset, I suggest that you power down the ONE SL that you have been struggling with and wire one or more of the other units to your router.

Describe your system and network for us. Which model router are you using? Are there any repeaters or mesh points? Network switches? What is wired? Wireless? Have you disabled the WiFi on any SONOS units? Note that some of the SONOS units will not use 5GHz.

I don’t know why this reply was duplicated, but I deleted the duplicate.

I was following advice that a SONOS rep gave to someone else on these boards about the same issue. 

I will try to work with the other device, which I did not factory reset. I still can’t imagine why they would have a factory reset that wouldn’t install like a fresh product. That seems ridiculously dumb. But I will work with the other device for now and be sure not to factory reset that one.

I am aware that some Sonos devices don’t do 5ghz, but the One SL does, according to Sonos.

There is a cable modem with no wifi capabilities with an ethernet cable right into the WAN on a TP-Link Archer AX73/AX5400. I actually used this router with a different Sonos system at my house, so I know it works. The router has 2x 5g and 2.4g networks, one of each for guests and one of each for me. I am trying to install the Sonos system on the 5ghz primary network. There are no switches, no mesh points, nothing complicated. It is an office building and there are other wifi networks in the vicinity with Sonos devices. But it seems to detect my devices during setup. It just won’t find the system or devices after saying setup was successful. 

Now, after unplugging the problem device and plugging in the other device (without an ethernet so far), I am getting alternating messages. It says it found my system. When I click “Join a nearby Sonos System” and then click “Continue” it says it found multiple systems on my WiFi network. When I click that second “Continue” button it comes back around and says “No Sonos System Found”. When I click “Done” it continues searching before coming back and saying “We can’t find a Sonos systom on <name of my WiFi network>. Make sure your sonos system is powered on and connected to the network.”

Now, with the other speaker which was not factor reset, I get this every time I am queued to type in my WiFi password. 

This is maddening. 6 hours now. 

All my other devices work fine with this router. I previously had a Sonos Arc on this router.

This is what the other device kept doing that led me down this rabbit hole to begin with.

The controllers don’t store anything critical. Try uninstalling then re-installing the controller.

That’s the problem. If you install a unit as “new”, it will establish a fresh “household” for that unit. A controller can only work with one household at a time. The controller is aware when there are multiple households on the network. With multiple Factory Reset’s, you probably confused the controller. During the fresh controller install, have only one household powered. After you are satisfied that this unit(s) function reliably, then you can power up and deal with the other units -- one at a time. If a unit belongs to another household, you’ll need to Factory Reset that unit and add it to the household.

I have managed to restore functionality to the unit which I did NOT factory reset, but only on the old router and network so far and on the old system. I am trying to go through the process of adding back the factory reset unit to the system. My thinking was, if I can get them back on the system together, then perhaps I can add a new wifi signal for my new router, to the old system. However, I am failing at both. The factory reset unit will not add on to the old system, even on the old router. The old system seems to not want to connect to the new router either.

Have you tried to set the old wifi name and password to the new router? This way a speaker you did not reset might not even know there's a new router…..

Factory Reset that Factory Reset unit again and add it to the system with the functioning unit.(s). I recommend wiring both this newly Factory Reset unit and a working unit while you do this. If you don’t have enough network ports to do this, wire the working unit. While you have the units wired, make sure that the WiFi SSID and password are correct.

As you swap routers it would be best to reboot EVERYTHING on the network. Yes, I know that you might get away with skipping this reboot 100 times in a row, but on 101 you can run into the issues that you are experiencing.

That’s a genius workaround.

What I find odd about these “wire your unit” recommendations is that it specifically prevents you from being able to set up the wireless. I finally just put it back on the old network. I keep doing the wired thing and it seems to help to keep one connected AFTER it’s set up.  But it won’t help DURING setup. I am still hoping to move it to the new network. Oddly, today is started having trouble with the old network and started prompting me to set it up on the new one finally….I am stuck back in yesterdays loop. It will detect it’s connected to ethernet and tell you to disconnect, but then when you do, it keeps insisting it’s still connected to ethernet.

Go to Settings → System → Network → Update Networks.

I tried that and it made things worse. It created more problems. 

I did that several hundred times. Plus the problem with that recommendation is it doesn’t work when it won’t even find the system.

I DID finally succeed after about 20 hours over multiple days. Here is what I did. Somehow, after moving back to my old router and restoring functionality to the one unit I did not factory reset, after about a day of using it with this one speaker on the old system, it finally detected the new router on its own which I had left plugged in for some odd reason. It just prompted me out of the blue on its own to see if I wanted to set it up on this new router after about a day of doing nothing but using it on the old system. Once I had done that, I managed to get the factory reset unit added back to the Sonos system without a hitch. However, I spent another day having problems with getting both speakers to work consistently. They seemed to work if they were plugged into ethernet, but Wifi would alternate between neither unit working, one or the other working, and one or both working on Wifi if a unit was hardwired with ethernet. So I finally decided to turn EVERYTHING on the entire network off (again...but this time closer to my goal) and do a reset on every device. I had done this before, but this time I did a hard factory reset on the router and reconfigured everything in my router settings to auto and factory settings while every device, computer, Sonos unit, etc were still unplugged and off. Then, i renamed the Wifi networks back to the names I had when I managed to change the network settings on the Sonos units when I got them back on the one system. Once that was done, I powered up the units and they both worked perfectly. I immediately deleted all the old network credentials from the other router so nothing would ever interfere again…..hopefully.

Moderator Edit: Language

While some very sophisticated enterprise level networks might tolerate multiple routers, the intent of home networking is that there is only one router. The “fault” was that the two routers and elements of the SONOS system could not figure out which router you intended to use long term. Some SONOS units were using router ‘A’, some were using router ‘B’ and the controller phone/pad was probably hopping between ‘A’ and ‘B’. Further, as you rebooted and Factory Reset this-n-that, units may have flipped between ‘A’ and ‘B’ or there could have been duplicate IP addresses.

In a word: chaos