The way it normally works is this:
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More on buying the key on Google Play
Google will store your purchase on their servers, forever.
If/when you switch or reset your device, please configure it with the same @gmail.com account (in system settings / accounts) as you originally used to to purchase the unlocker.
This will make my unlocker and other previously purchased apps show in the Google Play app (under My apps) as “purchased”, so you won’t have to pay again. You can use the unlocker on multiple devices the same way, by configuring them all with the same @gmail.com account.
Please install the base, free app first, and then the unlocker. Run the unlocker to activate the free app to “pro” status.
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You will need to add the same @gmail account in the device's system settings, under Accounts.
If you forgot the password, change it on Google's web site:
https://www.google.com/accounts/recoveryIf you are still having trouble with this, you can contact me at kmansoft / gmail.
I can create a free replacement license for you under any email address, in my own system, independent of Google Play.
The only requirement is for me to know any email address that you have configured in Aqua.