I actually had this problem with Nest thermostats: One of them was "locked", as in the whole UI was behind a password, by the previous owner, who moved out and took his wifi AP. I then moved in with a new wifi AP with a new SSID and new password, and the thermostat obviously couldn't get network access. Without network access it could not be unlocked remotely, and without being unlocked its network could not be reconfigured. There is no way to factory reset the device nor flash firmware to reset it. Eventually I called the previous owner and got a few candidate PINs (he had forgotten...)