I was having a similar issue, my modem was giving me issues with trying to do a firmware update and was doing one infinitely. I somehow got lucky and it let me use the internet after having restarted it several times but it was still saying it was doing a firmware update. I had to change some port forwards for a server I am hosting and it would not let me access the admin page so I had no choice but to fix it. After reading around and people saying neither netgear or the ISP could fix it I messed around with it a bit. I figured, if it has to update than it must have internet connection to do so. I turned off the modem/router unplugged the coax/ethernet and kept the power cable plugged in. Powered it back on with all that stuff unplugged. Then went into the admin page on my computer and logged in. THEN plugged in the coax or ethernet depending on your connection. And it stopped doing the firmware update
