I routinely have an issue where the Sonos Controller app (currently version 9.3) installed on my iPhone X (iOS 12.1) can't find any Sonos speakers on my network. Now since this has happened I deleted the Controller app from my phone, reinstalled, and tried to add my speakers back. Unfortunately when it gets to the point of finding the speakers on my wifi network, it can't. So now I have five speakers sitting here that I can't user because I can't get the app to recognize them. Extremely frustrating.
What could be causing this. I have gone through all of the troubleshooting steps, nothing is working. Very expensive investment in speakers to have sitting around unused.