Yeah, this one's common. Your laptop isn't properly connected to the router's network, or there's a browser cache issue.
First, check your physical connection. If you're using a wired setup, make sure the network cable from your laptop is plugged into one of the yellow LAN ports (1-4) on the router, not the blue WAN port. The router and modem should already be powered on.
If you're trying to connect wirelessly for setup, you need to join the router's default Wi-Fi network. The network name (SSID) is on the product label on the back of the router—it'll be something like ASUS_XX. Connect to that first.
Then, open a fresh browser window and type http://router.asus.com directly into the address bar. Don't use a search engine. It should auto-launch the setup page.
IF THAT DIDN'T WORK: Try a different web browser or clear your browser's cache. Also, ensure your laptop's Wi-Fi or Ethernet adapter isn't set to a static IP address; it should be set to obtain an IP address automatically (DHCP).