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/
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
So it appears this firmware 3.0.0.4.382_52745 was released by accident or has proved to be unstable on the RT-N66U's hardware & will not be re-released?