How to Find Serial Number, Imei Number Using iOS Sdk

Get IMEI and serial number from SIM on iOS programatically

Apple does not allow the use of private framework / api. Also, getting the users IMEI number will cause Apple to reject the app.

Some device identifiers are now impossible to be obtained from public APIs of iOS:

IMSI - International Mobile Subscriber Identity (SIM card number)

IMEI - International Mobile Equipment Identity (Device ID)

UDID - Unique Device Identifier for Apple iDevices

MAC address - Media Access Control Address (Network address)

Take a look here:
http://studyswift.blogspot.gr/2015/12/asidentifiermanager-get-idfv-vendor.html

If you could use any of the provided IDs the code is in Swift but if you use C# / Xamarin it won't be difficult to convert.

Hope this helps

You will have to keep UUID into keychain since keychain data persist after uninstallation of the app

iOS manages keychain information as sensitive information. you can use
SwiftKeychainWrapper library to easily access your keychain.

what apple doc says on SIM change: https://developer.apple.com/documentation/coretelephony/cttelephonynetworkinfo

CodingExample:

let uuid = UIDevice.current.identifierForVendor?.uuidString
KeychainWrapper.standard.set(uuid, forKey: "appUUID")

And whenever you want to get appUUID just fetch from keychain:

let uuid = KeychainWrapper.standard.string(forKey: "appUUID")

Note: You can also check if your UUID is changed after reinstallation
of the app and update it into a keychain.