The serial number is on the bottom left on the front of your Thinka next to the S/N. It is is a six character/number combination in captitals and looks like AB34GD.
It is NOT your HomeKit PIN, which is 8 numbers with dashes.
In case something is not clear or not working according to your expectation you can contact us, and we will provide instant feedback. We can be reached by email at support@thinka.eu, or by phone at +31-20-2420666
