I contacted the Yamaha UK Tech Support and got the response below
Make certain that you are running at least iOS 11.2.1
Try a different USB Cable
Please make certain that you are using a genuine Apple Lightning to USB adapter (Some 3rd party adapters may not work)
Delete the FM Essentials app from your iPad and then re install the app from the app store.
I’d already checked the above before contacting Yamaha Tech Support but still the problem persists
I'd successfully unlocked the app on my iPhone 5 using the same usb cable and a genuine apple camera adapter and my iPhone 5 is not running the latest iOS update.
The iOS on my iPad 9.7 which is just a year old is up to date running iOS 11.2.2 - I even deleted it from the iPhone then deleted and reinstalled the fm essential app on the iPad to check that it wasn’t working because of it being on more than one device - unfortunately the app still doesn’t unlock on my iPad. I would prefer to use the app on my iPad because of the larger screen and it's a newer device. Does this app only work on iPad Pro's and iPhone's and not the other iPad range?
One very important item to add to the checklist:
You must have an MX49/61 BK/BU/WH, or an MX88.
There are two different generations of MX keyboard, the App works on the second generation.
How do we know what generation we have?