From my Mac laptop client and from my iPhone 8, the only audio zone is “System Output” from the server. (I haven’t tested it but I’m assuming that if I turned on the stereo, that would appear as an audio zone.)
Restarting the server made my laptop appear as an audio zone but the iPhone still does not appear.
On the iPhone, the phone appears as a device and appears to be enabled. Pressing Disable does nothing and choosing Device Setup or its Load defaults option just sends the wheels spinning.