The fingerprint stuff is not correct in this one so it won’t auotmatically tie it to the device. I havn’t had enough time to understand how that works. I’m not sure Smartthings will automatically choose custom device handlers, maybe someone else knows?
But basically: Add the device (mobile app), add the device handler (web interface), edit the device to the new device handler (web interface).
It’s the type field (when you edit the device) that needs to change to the new device handler. Your custom device handlers will be listed at the bottom of the list when changing the type.
You don’t need a smart app to achieve a working device. That is only if you’re thinking about having some automation or rules tied to it. After adding the device handler and changing the device type it shows up in the mobile app with on/off and measurements.