Brief update, after rebooting the router for the 5th time, I was in luck and the “missing” Play:5 gen2 appeared on the iOS app and I could pair them as stereo pair. What happened then, you ask?

The fresh pair disappeared entirely and took every other speaker with it from the iOS app. On the Mac app only the stereo pair disappeared and the rest remained. on the web app everything, including the pair is visible. Surely, this is madness….. I have not yet again installed and re-installed the iOS app (but I clearly don’t want to keep doing this with every change I make to the system) also, if other iterations of the app can manage why not the iOS version?

also, when using the Apple Music app, I can send music to be played on the stereo pair as well….

I love my Sonos system but this becomes tedious.

Without more detail, it sounds like you may have set up two different ‘systems’, one with the ‘first’ group of speaker, the second with the pair of PLAY:5s. If you unplug the PLAY:5s from power, then open your controller, do you see the original system? if so, you’ll need to factory reset the PLAY:5s (likely again, as you would have to set them up as two separate systems), and then use the ‘Add a device’ process in your existing system and controller to add them back in. 

However, if this isn’t the case, I would 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 Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.