Sonos App shows either one device or the other, but never all together. Changes after restart.

Hello.

I own:

1x One. In room a.

1x Arc in room b

2x Era 100 in room c as stereo couple.

When I open the Sonos App in my iphone I see either room a and b, but not room c.
When I “kill” the App in the iphone and open it again, it shows me only room c, but not room a and b.

When I “kill” the App again and open it again, it shows again room and b only.

This means I can switch what I see every time I kill and restart the App. But I never see all rooms with all devices at once.

Any idea?

What I really want to do is to add the 2x Eras and the Arc in one room, but this is not possible due to the problem. I never see them together. I also don´t see them together when I put them in the same room on the app.

Additional info:

All rooms can be seen at the same time by my Apple Macbook. Also, all devices are visble at the Alexa App.

As Access Point and Router I use a Unify Dream Router. I already rebooted this router while the Sonos devices where unplugged and re-plugged them one after the other.

Also, the router shows all devices with their IP adresses. Everything looks normal to me.

Also, I changes some WLAN/WiFi settings in the Unify App like band steering and multicast. All without success.

Any ideas?

Thanks!