DIAGNOSIS The scooter's Bluetooth module has a weak signal or enters a deep sleep that breaks the pairing bond. It's a common glitch.
FIX Delete the scooter from your phone's Bluetooth settings. Delete it from the Segway app. Power cycle your phone's Bluetooth. Turn the scooter on. Open the app and pair fresh. Ensure you grant the app "Always" location permission on iOS, or precise location on Android. This is required for Bluetooth to maintain connection.
IF THAT DIDN'T WORK Interference from other devices can cause this. Do not keep a second smart device (like a smartwatch) very close to your phone when connecting. Power other devices off to test.
HEADS UP The scooter does not need a constant app connection to operate. Pairing is only for changing settings. You can ride without the app connected. Frequent re-pairing is annoying but not a functional fault.