I came by here because this is the most robust discussion of these cplay2air devices.
I too recently purchased a cplay2air device and had a hell of a time getting connected.
I would get completely connected, everything looked good, add my wife's phone - worked good...
But then the next day she'd leave and it wouldn't work.
After some thought (and maybe this has been covered here) I had a lightbulb go off.
I submitted this to cplay2air support and they confirmed all the below:
1. To use cplay2air, the phone's WiFi must always be connected to the dongle. This means, while using cplay2air, your phone will not use vehicle hotspot or other WiFi, only LTE
2. If car is in a driveway and connected to house WiFi, initial start will cause confusion because phone will be on house WiFi and not connected to dongle. When you drive away - the phone might not auto-connect to dongle WiFi.
3. If phone is configured to auto-connect to vehicle hotspot, you may never connect because the phone will not be using dongle WiFi - preferring to use the hotspot instead.
In my case, I would - through following the steps and checking firmware, etc... - I would be connected properly via the dongle WiFi. I would get everything configured and working. I would go back to my "normal" configuration which is, at the house - house WiFi, in the car - car HotSpot. Which means I never connect back to the dongle. And thus I never got the thing to work.
On the plus side, knowing this allows a user to easily control if they want to use CarPlay or not by simply changing their WiFi connection.