So, after pairing this device, and it successfully working, you turn off Bluetooth on the phone and then turn off the X2? You then turn the X2 on, and then turn on Bluetooth on the phone and it will not connect, even though the X2 is listed in Bluetooth devices on the phone? Is this correct? Have you experienced any issues with Bluetooth on the phone with any other Bluetooth devices?
If that is correct, and you have not experienced any other issues, I would check the documentation for the X2 to see how to reset that device. Make sure you remove the X2 from the phone list before turning them back on again. Once you have reset the X2, put them into discovery/pairing mode and then turn Bluetooth on for the iPhone. See if you can successfully pair the two. If you can, then try turning them off and and back on again. If you cannot connect, then contact their support to see if there is an issue with using them with the iOS devices. If they are successfully pairing, then it shows the Bluetooth on the phone is working properly.