How to register Kindle App on old iPad (Solution)

I recently de-registered the Kindle App on my 1st Generation iPad so I could re-register it with a different Amazon account. Unfortunately, when I tried to re-register it with Amazon, I got a "Device Registration Failed - No customer found for this e-mail address and password" message. Then I received a text message on my iPhone from Amazon providing a security code.

The old app on my old iPad didn’t know how to handle the 2-factor identification. So I called Amazon.

Solution: Enter your Amazon username and password in the Kindle App, just like you normally would to register the device, and click the Register Kindle button. When you receive the security code on your phone, re-enter your username and password in the Kindle App on the device you want to register - this time adding the security code directly after your password. For example, if your password is happy and the security code is 123456, enter happy123456 as your password (no spaces).

Hope this saves you some frustration.