WardenCam is same as all other apps you purchase on Google Play. It is linked to the google play account with which you made the purchase.
Here is the procedure to activate premium upgrade on all your other devices.
If you purchased on one device and like to enable the upgrade on another device of yours, you need to use the following steps.
1. make sure the primary account you used to make the payment is synced and the only google account on the second device
2. start wardencam and confirm you see this message "upgrade is purchased already, please sign in with the licenced account"
3. sign in with your licensed account as viewer once then you should see the upgrade enabled on the second device.
For example, I use
test001@gmail.com (primary account) to purchase the upgrade. In WardenCam I sign in with
user001@gmail.com (licensed account, can be the same as primary account)
To enable upgrade on my second device, I need to first make sure the
test001@gmail.com is the only google account in sync on the device. Then use
use...@gmail.com to sign in to WardenCam.
Please let me know how it works for you.