For the benefit of anyone following this issue...
The app isn't supposed to connect while the desk is paired to the Hue hub.
If you want to add anything to the hub it has to be on a network that has Internet access. The network you have your desk on should NOT have Internet access.
It follows, therefore, that you need to connect the hub and the device with the app on to a different network to add devices.
If you want to use the app for other reasons while the hub is on your lighting network then obviously your Apple or Android device needs to be on that network. You may get a message warning you that there is no Internet - do not ignore this as you may have to accept it to get connected properly and for the app to work. You also have to unpair the desk and hub.
Be methodical and patient and it should work in the end. Try not to be in the position of connecting the app while under pressure, do it before you are in in trouble and it should all just work when you need it.
Don't forget to re-pair the desk and hub when you are finished with the app.
This should be a nice addition to the solutions available but it is a marriage of theatre and domestic technology and your mindset needs to accommodate this.
Have fun!