Recently Spotify Connect devices (three different Teufel speakers) disappear from the selection menu. I have done lots of troubleshooting on the speakers and the network, including full resets. Nothing helped. The only thing that works 100% of the time is uninstalling the app from
the phone (deleting all data) and installing it again. This was the “fix” three times now, but it’s not a good one, as I travel a lot and I always lose the offline music and have to re-download it.
To me, it seems a problem inside iOS or the Spotify App for iPhone. The speakers appear on the iPad and on the computer (Windows 10), even when they are not visible on the iPhone. So, I would rule out the network and speakers as a root cause.