I just successfully installed the following firmware yesterday on a Netgear N600 WNDR3400v3:
The very first thing I did was clear the NVRAM.
Afterwards, when I disable the 5GHz wifi but keep the 2.4GHz wifi enabled, the router's front blue wifi light gets turned off. If I reboot the router, the blue wifi light still remains off.
I don't know if this is intentional, but that blue LED seems to be purely tied to the state of the 5GHz wifi exclusively.