I have both a Nexus 7 tablet and a Nexus 5x phone. With the device cable already connected to the controller, I first start up the Nexus 7 screen, followed by the remote and then the drone and I get a prompt (in my case to choose between DJI GO or Litchi, as I have both installed).
However, if I use the same sequence with the Nexus 5x, the device will not prompt until I unplug and re-plug in the USB device-to-controller cable.
I don't know why there's this difference, possibly it's because the Nexus 7 is on Android 6 and the Nexus 5x is on Android 7. It doesn't matter anyway, as I now know how to do it on both.
So I suggest plugging in your USB device-to-controller cable last - and see what happens.