What do you want to achieve exactly? If you're going to use this router as I described it, it will perform only as an access point, another device in your LAN, it won't have direct connection to the Internet and it's gonna use your ISP router as a gateway. If you want this ASUS router to be the central point of your network, you've got to connect the cable from the ISP router into the WAN port and:
Disable firewall, DHCP server and WLAN on your ISP router
If you have dynamic IP, select IP from the LAN subnet of your ISP router for the ASUS router and enable DMZ for this IP
Configure your ASUS router. If you have dynamic IP, for the WAN port set the IP you chose in the previous step. If you have static IP, you should be able to simply configure WAN port with the same settings as the WAN port in your ISP router.
In this case scenario, your ISP router will act only as a "modem", and your ASUS router will manage your whole network (including firewall, routing, etc.).
If you will use ASUS router as AP, the 1Gbps connection will be only possible between devices connected to the switch of this router (other LAN ports), but the rest of the network will be limited to the 100Mbps link between both of your routers (and WLAN speed is another thing that will also depend on the specific scenario).