You need to turn off the routing function in the Asus router, make it into just a wireless access point. Log into the router with a web browser via it's IP address. If you haven't done this before, follow the instructions that came with it for setting it up. You may have to disconnect it from your ISP's modem/router in order to do this, so you may have to just connect a computer to the Asus router. It's default address is probably 192.168.1.1, but the user manual will tell you that, along with the default userid and password.
The manual may describe this in detail, but if not what you want to do in general is to turn off the Asus router's DHCP, so it will not hand out IP addresses. You want to turn it into a wireless access point, or a switch, so it passes all requests for IP addresses to the ISP's modem/router.
The next thing to do is to manually give it a static IP address. Assuming your ISP's modem/router is 192.168.1.1, then give the Asus router an address of 192.168.1.2 (or something else you will remember and that is not being handed out by the ISP's modem/router's DHCP function).
You may have to disable any routing functions, I don't know which screen that is on with the Asus router, but it shouldn't be hard to find. Then again, disabling DHCP and giving it a static IP may be enough.
Then connect it back up to your ISP's modem/router, BUT DO NOT PUT ANY CABLE INTO THE "WAN" or "INTERNET" port on the Asus router. Just use one of the LAN ports to connect to the ISP's modem/router, and any computers or switches you connect to the ASUS should go into a LAN port. When you're not using the Asus for routing, NEVER plug anything into the WAN/INTERNET port of the router.