Issue:

You have an ASUS modem Wi-Fi router.
You already configured it in the past. Now you want to reconfigure it but you cannot access it via http://router.asus.com/.
You already tried to restart the router several times but the problem still persists.

Solution:

1. Perform a hard reset of the router.
2. Connect to the default 5G Wi-Fi network of the router.
3. Go to http://router.asus.com/.
4. Check and update the firmware.
5. Reconfigure the router.

Note: If you configure the LAN information of the router manually then remember to set up an IP address for DNS Server1 (e.g. 8.8.8.8)  and DNS Server2 (e.g. 8.8.4.4).