Did you change anything? Might anything have received an update or restarted in some way?

Unplug all your Sonos gear from power. Reboot your router. When it’s fully restarted and wifi running, restart your speakers starting with whatever is plugged into your router, and waiting for each device to restart before powering up the next. 
 

This fixed it last time you had this problem. I’m guessing you haven’t yet set fixed ip addresses for your speakers?

You’re right, unplugging all the Sonos gear, rebooting the router, and restarting the Sonos units worked for me in May.  Thank you for your help, then.  And now.

This time, I‘ve unplugged everything, rebooted my router, and tried restarting my Sonos Boost.  I’ve done all this a few times, sometimes turning my phone off and restarting it as part of the process, sometimes deleting and restarting my Sonos app.  I have the Plume WiFI app and it will often show my Boost as online (as I’m writing this reply it was online at the start, and now it’s not).  And when I’ve reloaded the Sonos S1 Controller app it has indicated ‘Connected to your Sonos system’, but when I press Done it says ‘Unable to connect to Sonos Learn More’.  At one point the phone app directed me to push the ‘infiniti button’ on the side of the Boost.  Which I did.  And then I got the ‘Unable to connect to Sonos Learn More’ warning.

You helped me in May.  Maybe you can again.  Thank you.

Have you recently upgraded to iOS14?

I did get a new iPhone recently.  I think that would mean, yes, I recently upgraded to iOS14.  Sonos was working with the new phone, until a week or so ago.

If so this thread might be relevant 

https://en.community.sonos.com/troubleshooting-228999/s1-app-on-iphone-not-showing-my-sonos-devices-6849336

So, I confirmed the settings on the Sonos S1 Controller app were enabled for Local Networks.  I deleted the app and reinstalled the app.  I unplugged my router, my Sonos devices and turned off my iPhone.  I turned the router on again, and then my phone and then my Sonos Boost.  My iPhone Controller app at first indicated I was connected to the Sonos system, but when I advanced to the next window I again got the ‘Unable to connect to Sonos Learn More’ warning.

Anyway, no change...and no luck.  But I can confirm the Local Networks setting is enabled.

Any other suggestions?

I have exactly the same issue since yesterday. The App can’t find my Sonos system.

I have verified the Local Networks access on my iPhone (iOS14). I have re-installed the app, rebooted the router etc. No luck.

The next thing I would try as a troubleshooting step would be to wire a Sonos device to my router (if I didn’t already have one wired) and to wire a different Sonos device (if one already wired).  Give it a few minutes to reconfigure then post back with the outcome.

Thank you John B - that did the trick! 

I have a system with 4 different speakers and a Connect. I took one of the speakers and wired it directly to the router and lo and behold, suddenly my App could connect to all my devices!

One possible cause is that your phone was on the router’s 5GHz band, Sonos 2.4GHz, and your router doesn’t bridge the two (maybe).  If you can force your phone to connect to 2.4GHz you may find that you can disconnect from Ethernet.

But if it is convenient for you to keep a speaker wired then I would recommend you do so.

Sonos failed again !!!

It seemed to work a couple of hours ago as the App found the Sonos system again. Now that I want to play some music, it bugs out the same way again ...

My Sonos Boost was wired to the router, but I tried connecting a Sonos Connect to the router and rebooting everything.  No luck.  So I connected a Sonos Play:5 to the router.  And I have music!

But only through the Sonos Play:5.  The Sonos Connect appears in my Plume WiFi app as connect,  it the (now disconnected) Boost doesn’t show up.

What do you make of that?

Do you still have a speaker wired?

I had disconnected the speaker again. In frustration, yesterday evening I drew a cable between the floors of our house to permanently hardwire my Sonos Connect. That solved the problem yesterday, we had music. Today however, it’s all the same again!

Try unplugging all Sonos devices from power, then reboot your router. When the router comes back up, plug back in all Sonos devices.
 

Note that all Sonos devices must be unplugged during the router reboot, otherwise you’re only getting some, not all, of the benefits of the procedure. 

I’ll try that, but at the same time I’m wondering why this happens now, and repeatedly? I’ve had the exact same system with the same router several years now, and never had this issue. Nothing has been changed, it happened out of the blue ...

Ok, it works again (just like yesterday), but what about tomorrow? Will I have to go through this procedure every day now???

@Ziggy66 : Try setting fixed adresses for your Sonos units in your router’s DHCP table.

When done, refresh your system as descibed above by Airgetlam.

This happens to me a lot! I mostly get satisfaction when I restart my eero network, but sometimes it just doesn't connect. I’ll give up and return the next day to see it miraculously working again...until the next time. I get tired of playing these games and only want my system to work more often. Ready to try another ecosystem.

I fixed this problem by “bridging” my eero in advanced settings.