I forgot the password, or WiFi is inaccessible, how can I reset the device to factory defaults?

For the correct procedure, see the PowerCable xxx QIG (Quick Installation Guide).
It is available for download at this page as well as included in the product package.

To restore factory defaults, hold the button pressed, plug the device into a mains outlet and keep holding the button for at least 10 seconds (Green LED quickly flashing for 1sec). The device reverts to factory settings and the AP mode is activated (Yellow LED blinks quickly).