I recently acquired a bricked ASUS RT-N66U. It is most definitely bricked and refuses to take any firmware loads or allow any communication whatsoever.
After chasing down the usual couple hundred or so rabbit hole dead ends on the Internet (which includes ASUS support) I found a solution and it worked for me. It may work for you.
What is needed is the ASUS Firmware Restoration Utility, which, according to some sources, ostensibly, works on all ASUS routers. I cannot confirm that claim having used it only on the RT-N66U.
I finally found the utility download via this thread --
Here is a direct link to the pertinent download --
For some reason ASUS has decided not to include this utility on the RT-N66U support page, but it turns out that if you go to the RT-AC68U support page the utility can be downloaded from there -- https://www.asus.com/us/Networking/RTAC68U/HelpDesk_Download/
Preliminary: Download and install the ASUS Firmware Restoration Utility. Download the ASUS RT-N66U firmware from their support page -- https://www.asus.com/us/Networking/RTN66U/HelpDesk_Download/ Connect the router to your PC via a UTP Ethernet cable. (Disconnect any other wireless or wired connections you may have to your PC.)
Step 1) Hold down the reset button and power up and keep holding it down for 30 seconds (some resources say 10 seconds but I held it down for 30). This step, ostensibly, puts the router in "rescue mode" according to various sources and the power lamp will blink slowly. The router given to me was, evidently, already in this mode, but I did this step anyway.
Step 2) Launch the ASUS Firmware Restoration Utility and install the firmware.
It will take several minutes to complete the firmware load.