This is a very short description how I upgraded my ASUS RT-N66U router. My router had a CFE bootloader version 188.8.131.52. This version only supports 32K NVRAM, which was not enough for my DD-WRT configuration.
Disclaimer: Following this steps can brick your router. If you are unsure, don’t use this description!
Hard Reset will clear out the NVRAM. For this router use the WPS button, not the reset button.
I followed the original description:¨CFE bootloader update¨.
I installed the Fractal Build version 20202 mega for RT-N66U with CFE upgrade via ASUS Firmware Restoration tool. This Firmware Restoration tool is part of the ASUS RT-N66U B1 Utility (choose your OS -> Utilities).