Same thing is happening to me.  I have 10 devices. Very frustrating. 

If you have not tried yet, connect one of the Beams to your router with an ethernet cable. Many have regain access to their devices by doing this.

I have the same problem since a couple of weeks. Tried multiple factory resets ans Connecting via cable. I can see on my router that the Sonos is connected. But it won’t show up in the app. When I reset the app and say: “available device”, the app even finds it. But after that it won’t show up in the app. 

IP is fixed on the router for the Sonos.

I even reset the router to factory settings and reconfigured everything.

Tried to turn private mode on and off on my iPhone. Didn’t do anything.

S1 App says device incompatible. (But doesn’t let me remove the device)
 

sonos beam gen 1. (Only device)

iOS S2 App

Have you done an IP reservation for devices you want to assign a static IP to? For DHCP, I started my IP address at x.x.x.75 and set address for my speakers below the .75 address pool.

As I wrote, yes I did. But still not working.

Can you try the desktop app to see if it shows up there?

Sounds similar to the issue described here 

I just installed the desktop app. It can find the Beam, but I can’t do anything, cause it’s apparently not configured.

But I went through the whole installation- and configuration-process after the factory-reset with the mobile-app. After that, the message with disconnecting the sonos-device from power and connecting it again to show up in the app popped up. But it won’t appear in the app.

This is the excert from my Fritz!Box: 

I just solved it:

I did another factory reset and an app-reset.

Instead of doing: “Add an existing device” I did “Set Up new Device”.

Went through the whole process. And voilá it’s in the app again!