What is the make/model of the new router? …and if that router has ‘QOS’ or ‘Airtime Fairness’ enabled, then switch off those services within the router configuration pages. (See your router manual, if necessary).

I would power off the speaker that is working (just temporarily) and wire the ‘broken’ speaker to your router and then just see if it appears in your Sonos App, if so, add the new WiFi settings to your speaker, via ‘Settings/System/Network/Manage Networks/Update Networks’ - oh and also remove your old routers WiFi credentials in that area too whilst making such network changes (that’s if you haven’t done that already?)

Once you have disabled QOS/ATF on the router, then uncable the speaker, having updated it’s WiFi credentials and then power on your other working device - all being, well both ‘should’ appear in your Sonos App.. if not, then there is a chance you may have caused yourself an issue, by earlier factory resetting the one device, but let’s cross that bridge later, if you perhaps discover that is the case and each device is running in what is often termed a completely separate Sonos Household.

Report back here if you think that may have happened after completing all the steps mentioned above.

Hi Ken,

Many thanks for the prompt answer!!!

The new system is a DECO M4 Mesh system (TP-Link product). Honestly I don’t know whether it has QOS or not. I checked the settings but did not see anything like QOS.

I have tried to follow the steps in your answer. Now the application doesn’t see the ‘good’ speaker at all, however it sees the ‘bad’ speaker, althogh it cannot add the speaker to my account. Actually nothing can be done with the ‘bad’ speaker through the application, and the ‘good’ speaker disappeared.

Funny thing that in the meantime I have downloaded the Sonos software to my computer and it can use the ‘good’ speaker. While the mobile application cannot use any of the speakers. :((

The other interesting thing that on the Sonos webpage under my account I can see both speakres, but only the ‘good’ speaker is indicated as ‘online’. The ‘bad’ speaker is not indicated neither as ‘online’ or ‘offline’. 

I wish I could understand why it is not possible to clear everything from the speakers and start again the registration from the very srcatch… It would be much easier.

Okay, perhaps try this .. wire the first (good) speaker back to the primary tp-link deco hub and fully close (slide off screen) the Sonos App on your mobile device. Power-off the other uncabled speaker.

Re-open the Sonos App and you should see the one wired Sonos device, (hopefully).

Then factory reset the ‘other’ uncabled speaker by following the instructions in this LINK 

When the status LED is flashing green on the reset speaker, then go back into the Sonos App and goto ‘Settings/System/Add Product’ and setup up your reset speaker again by following the instructions on screen and see how that goes for you.

Once the devices are setup and working (all being well), then see what happens when you then uncable the speaker from the tp-link hub. If either speaker drops from the App when unwired then report back your findings here, but don’t opt to reset anything further, just cable the first speaker back to your primary Hub.

Hi Ken,

I did my best, but nothing has changed. The lost speaker is still lost. And the good speaker does not work. 

I have contacted the Sonos Support. The first attempt has failed. The next step will be a remote access to my system. Hopefully they can do something to make my speakers alive again.

Many thanks for your efforts!