To reset the router to factory settings start with the router powered on and look for the hidden reset button on the bottom of the router, it is the red button recessed in a round hole to the upper left corner of the silver label.
Using a pen press the hidden button for 5 seconds and then release. You will see the WLAN light on the top of the router go off, wait for it to come back on and your router has now been reset.
The reset button is highlighted with the red arrow in the picture below: