After a router reset, reboot everything on the network. You’ll have less overall work if you reuse the same WiFi name and password. After the reboot, go into the router configuration and “reserve” IP addresses for all regular network clients.

I don't keep my speakers powered on all the time. The WiFi config hasn't changed since it worked. The router was just powered off and on again.

Since you power off your speakers and power them on when you want to use them, other devices may grab the IP address they the speakers had. This will cause connectivity problems.
 

As ​@buzz has stated, creat an IP reservation inside your router’s settings and assign static IPs to all your speakers. This way when you turn them on again, they will grab the same IP address every time. This is how I have my speakers corp my network. I do not have any connection issues.

Thanks for the feedback. I checked the router IP definitions and they are all reserved already. I can see the 2 x SONOS one and 2 x PLAY:1. I restarted the router to check the IP addresses and they were the same. 

I then downloaded the SONOS windows app and it eventually discovered the 2 x PLAY:1’s and did a software update to each. I can see them online with the router, but the SONOS mobile app cannot find them I have hard reset them multiple times. I have even installed the sonos app on an older phone running an old version of Android. The result is the same. I cannot find them with the app. Is there anything specific about the PLAY:1 that I need to consider? As i said in my original post, everything was working fine before the router power off and restart. I now have the 2 SONOS ones consistently available. But nothing I do seems to make the PLAY:1’s available, despite them being online and visible in the router with the correct IP...

Have you tried plugging them in to your router for a while?  

I did connect them via cable.. same result. The router does show them as available... 

Make sure that the router’s configuration allows data to crossover between the 2.4 and 5GHz segments.  

Both the SONOS One and PLAY:1 speaker pairs are using 5Ghz. Connection to the SONOS Ones is almost reliable, but I just cannot add the PLAY:1’s. I can see them in the Router device table. I have used a SONY phone and also a Google Pixel 6. Neither of them will find the PLAY:1’s… 

When you are near one of the SONOS speakers, momentarily switch to Airplane mode.

Tried swapping to Airplane mode briefly - didnt make any difference. The app on Windows or either Android phone cannot find the PLAY:1’s, even though the router shows all 4 speakers online, available and on the same network (5Ghz). It shouldnt be this difficult - especially as before the router power off/on everything was working...

Ok, I have had enough. SONOS lost the config for the SONOS One’s and when I tried to re-add, it errored. Its time to walk away from this for a while. I have wasted so much time on this since the rogue release of the app….time to look for alternatives