This model was end of its life, and its firmware, utility, website, and manual will no longer be updated. For more details, please refer to https://www.asus.com/event/network/eol-product/
Asuswrt-Merlin is a third party alternative firmware for Asus routers, with a special emphasis on tweaks and fixes rather than radical changes or collecting as many features as possible.
Go to Wireless settings and set both 2.4GHz and 5GHz bands to Fixed Channel. 2. Reboot the router. 3. Proceed with the firmware update. 4. After the firmware update, the 2.4GHz and 5GHz channels can be set to either Auto or Fixed, depending on your prefer
Yesterday I upgraded the RT-N66U to the latest firmware and it is back in place to serve his iPhone, Windows computer, MacBook, Play Station, Smart TV, his friends phones when they visit.