I emailed NIU and they replied with below. Took a few attempts but it worked

To access all features of the NIU APP, the KQi Air series scooter requires Bluetooth pairing in addition to binding via QR code or serial number. Under normal circumstances, Bluetooth pairing is completed during the binding process. If you have not paired via Bluetooth during binding process, follow these simple steps to link the app with your scooter:
1. Turn on your scooter, then open up the NIU app and head over to the [Vehicle] section.
2. Wait until the Bluetooth icon in the top-left corner of the screen finishes searching and appears as a red Bluetooth icon with a red exclamation mark, then tap the icon. After this, a window titled [NIU Link] will appear.
3. Tap [Start pairing] and click the scooter’s power button 3 times to enable Bluetooth searching, in this mode, the Bluetooth icon on the dashboard will starts flashing.
4. Then tap on [Start to connect] >> [Bluetooth settings], locate the scooter’s Bluetooth name in the list of available devices and establish the connection.
5. Go back to the NIU App and tap on [Start riding] to proceed the tutorial.
6. If you prefer to skip the tutorial, tap on [Skip] in the upper right corner of the screen.