With DJI RC-N3:

Connect your mobile device (connected to the internet) with the remote controller, and then connect to DJI Air 3S via the DJI Fly app. Follow the instructions in the app to activate DJI Air 3S. With DJI RC 2:

When using DJI RC 2 for the first time, connect to the internet via Wi-Fi or a smartphone hotspot, and then follow the instructions in the DJI Fly app to activate the device. After activating DJI RC 2, connect it with DJI Air 3S via the DJI Fly app, and then follow the instructions in the app to activate the aircraft.