DIAGNOSIS The solid red internet light means the router can't talk to your modem or get an IP address from it. It's usually a simple handshake problem between the two boxes.
FIX First, power cycle everything. 1) Unplug the router's power. 2) Unplug your modem's power. 3) Wait a full 60 seconds. 4) Plug the modem back in and wait for all its lights to settle (usually 2 minutes). 5) Now plug the router back in. Give it another 2 minutes to boot and negotiate. That red light should turn white.
IF THAT DIDN'T WORK Check the physical connection. Make sure the Ethernet cable from your modem is plugged firmly into the router's yellow WAN port, not one of the blue LAN ports. Try a different cable if you have one handy—cables do go bad.
HEADS UP If you just switched from an old router, your modem might be locked onto that old device's MAC address. A longer modem reset (hold the reset button on the modem for 30 seconds) often clears that lock and lets the ASUS connect.