You must have received a QR Code with your activation and you must be connected to a mobile or Wifi network with internet access.
From an Android device:
Tap Settings > Network & internet > Mobile network > +
- Follow instructions.
From an Apple / iOS device:
- Open the Camera app and scan your QR code.
- When the Cellular Plan Detected notification appears, tap it.
- Tap Continue, at the bottom of the screen.
- Tap Add Cellular Plan.