Further information 

router make/model is Nokia Beacon 2, in bridge mode. The primary DHCP server runs OpenWRT with the firewall on.

App is running on iOS on an iPhone and an iPad, both at the latest version 17.5.1

tomorrow I will check that the Nokia can see the Playbar. 

I still cannot get the Sonos S2 app to see the Playbar.

Nokia can see the Playbar. The Nokia app shows the Playbar MAC address and displays it as SonosZP with a valid IP4 address, at 2.4GHz.

SONOS updated their controller app about 5 days ago, so I tried again. Secondly, I read several helpful suggestions from Ken_Griffiths. This is what I actually did. It’s possible that I need not have done some of these steps.

1. Update the app on my phone.
2. Plug the ethernet cable into the Playbar. (Never remove it.)
3. Reset the Playbar (unplug power cable, hold Pause/Mute button while re-plugging power).
   1. Light starts white then goes red.
   2. Light goes flashing green to show it’s done.
4. Open Sonos S1 app and look for the system.
   1. Log in to Sonos.
   2. Searched for system.
   3. It found the Playbar and recognised it as an S2 appliance.
   4. It suggested going to the S2 app. Clicking on that option took me to the new app without having to do anything. ie no need to log in or out etc.
   5. Sonos (s2) app proceeded to attach the Playbar, prompt me to attach it to the TV, and tune it.
   6. Once that worked, I was able to attach the Sub (gen3).
      1. Tuning the Playbar and Sub at that point did not work. “Something went wrong. Please try again.” I tried once more, but it failed again.
      2. At this stage, all was working well enough that I decided to stop.

The system is working well enough. I suspect that the newer version of the app was crucial, as was maintaining the ethernet connection to the Playbar.