I am extremely surprised that box even has asus name on it. I figured this was another question from someone who was too lazy to read the manual. Even with a lot of searching all I found is their worthless quickstart guide that says push magic button and it works.
Asus has some of the better documentation but this product it is almost worthless. Many of their routers you can run a simulated router online to see their menus before you buy the products.
It should be a matter of connect a lan port on your router to one of the ports on the device and then set it to AP mode. Where you find that magic AP mode setting is hard to say.