First of all, make sure all other drone related apps are FULLY closed. If the light on your controller is green it means it is linked to the aircraft. At that point you should be able to connect to your aircraft with the app as long as the app has been registered.
You need to open the layout screen at least once while on the internet so the app can register with DJI. It will say "Registering" at the top until this happens. It will not connect to the aircraft until it says "Looking for Drone".