How to access the Asus RT-AC66U router? The web address that you are using to access the web page of your Asus Router should be accurate. If the Asus rt-ac66u setup web address is not working, then you need to use the IP address. Inaccurate spellings, IP addresses, and login credentials can direct you to some other web page instead of the login page of your device. Check the distance of your Asus router with the modem.