This is intentionally impossible. You are not permitted to track the user this way. If you attempt to work around this restriction, Apple will likely break your technique in a future release (they've quietly broken several hacks people have used previously, generally without warning), and they may reject the app entirely.
The correct way to identify a device is with identifierForVendor
. It is reset to a new value whenever the last of your apps is removed from the device and one of your apps is reinstalled. (By "your apps" I mean apps with the same start to their bundle id.) The fact that it can be reset is a system feature, and Apple does not allow you to circumvent it.