Problem description: We have recently discovered a bluetooth bug on iOS 11. Sometimes happen that user is not able to connect to drone even though the drone is visible in search window in app.
Solution: Follow these steps to solve the problem.
1. Turn off bluetooth
2. Turn off device by holding turn off button for a few seconds.
3. Turn on device.
4. Turn on bluetooth
You can downgrade your iOS device to 10.x.x version until Apple fix this horrible bug. Link here