DIAGNOSIS You're locked out, and the only way back in is to wipe the router back to its factory-default state. This erases all your custom settings, including your WiFi name and password.
FIX 1) With the router powered on, locate the small, recessed 'Reset' button on the back. 2) Use a paperclip or SIM-eject tool to press and hold the button down. 3) You'll see the power LED start to blink. Keep holding until the LED goes out and then comes back on solid (this takes about 8-10 seconds). Release the button. The router will reboot with factory defaults.
IF THAT DIDN'T WORK If the button method feels unreliable, you can try the power-cycle reset: Unplug the router, press and hold the reset button with your paperclip, plug the power back in while still holding the button, and continue holding for another 10 seconds after it powers on.
HEADS UP After the reset, the default login is username 'admin' and password 'admin'. Your WiFi network name will revert to something like 'ASUS_XX'. You'll need to set everything up again from scratch, including your internet connection type (PPPoE, DHCP, etc.) if it wasn't automatic.