|Click on my name above in my response and read my how-to guide titled "Adding a second router.|
You'll want to pay attention to the version where you are extending your LAN and not the one where you are adding a second, separate subnet.
From what I'm reading you can't use the ASUS device as a modem if it does not in fact have a modem built into it. The modem is the device that converts the signal from the phone to the network. I suspect you'll have to add the ASUS as the second router. However, this doesn't prevent you from creating the wireless LAN on it. If you just don't create the WLAN on the other device, then only the ASUS device will be providing a wireless network.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.