I think you can follow 2 approaches
1) Connect the asus directly to internet and let it handle routing functionality.
(the asus has a built in modem, for which you have to configure you internet settings, and connect the 'internet' to the WAN port)
2) let your current modem control the routing and set up the asus as a 'switch' (For this, you have to set up the asus as 'Access point only' in the system configuration. But you will loose all routing capabilities off the asus if you do this. However, you gain an extra LAN-port, because the WAN port is in this setup threated as a normal LAN-port.
btw. this is well-documented in the manual
Hope this helps.