Nighthawk Nightmare - won't factory re-set (tried the 7 second and 30 second button-press...)

https://kb.netgear.com/000059634/How-to-upload-firmware-to-a-NETGEAR-router-using-Windows-TFTP
Need to press the reset button for 30 seconds then release, then it send to PUT the command and upload the FW. 
Make sure you enable tftp: https://www.thewindowsclub.com/enable-tftp-windows-10
Video walk through using a tftp client. Use the built in one for windows 10 though. the video is more of an example of how its done:
https://www.youtube.com/watch?v=R9DLrLQrpaw
I had a problem on a Windows 10 PC where this didn't work to use TFTP, so I switched to a Windows 7 PC and the TFTP worked.
Mac TFTP: https://www.petenetlive.com/KB/Article/0001247

Try nmrpflash:
https://github.com/jclehner/nmrpflash

I recommend loading v.112. There seems to be problems with v.120 currently.