To provide you more detailed instruction, you can also click ASUS Youtube video link below to know more about "How to Hard Factory Reset ASUS router"
1. Turn the power off.
2. Press and hold the "WPS button" and turn the router on.
3. Power light is on (keep holding the WPS button).
4. Continue to press and hold the WPS button.
You see the power LED ligh on(white) -> power LED light turn off -> power LED light flashing green color -> power LED light turn off and then you can release the WPS button.
5. Reboot your router by manually pressing the power button.
Find the WPS button on your router, kindly refer to the following picture.
Different models might have slight differences. If you have any question, please check the user manual
The following picture takes [ZenWiFi_XT8] as an example.