change owner (associate) of smart thermostat

Hi everyone. I have an issue when trying to connect to my smart thermostat. 
1. When on PC, I can connect via USB cable, but I can't associate the product with my account, because it says that product is already associated with an account.
2. When trying to discover it via app (iphone), it's never discovered. 

I bought the house where it was installed, and previous owner (older gentleman) has no memory of ever setting that up.
How can I reset previous owner without him? 

Thanks!