Could you please:
1) Go to Android Settings > Security & Location > Device admin apps and disable Prey as one of them.
2) Go to Android Settings > Apps & notifications > App info > Prey > Storage and clear data.
3) Go to step #1 and reenable Prey as an admin app.
4) Open the Prey app again. It'll ask for your email and password, and that will get the device added to your Prey account once you wish the process.
(Instructions could be a bit different depending on your Android version.)
The problem is caused by Google's cloud backup, which restored the config file from your old Android device into the new one. These steps will force the new unit to be added.
Please let me know if this solve the problem
Have a nice day!