If your router is unresponsive, you can trigger this emergency server to manually upload new firmware: Asus RT-N66U only ASUSTeK - CFE miniWEB Server works
Because this server is , it is not a standalone software you "download" to install. Instead, you access it to perform firmware recovery when a router is "bricked" or unstable. How to Access the CFE MiniWeb Server cfe miniweb server download