Cannot change DNS settings (R6120)

> [...] I get a "403 Forbidden message". [...]

   Bad/corrupt firmware/settings?

> I'm not using the SSO login [...]

   I don't know what that means.  These devices don't have a single
sign-on authentication scheme, only a simple user-name/password scheme,
not shared with anything.

> [...] (simply try to login 3 times and it will finally let you into
> the controls).

   Perhaps it's not really letting you in so far as you think that it
is.

   I'd do a settings reset, and manual (re-)configuration.  Get logged in
properly.

> My firmware is V1.0.0.66.

   Try a different version?

   Visit http://netgear.com/support , put in your model number, and look
for Downloads.  (For older versions, under Firmware and Software
Downloads, look for "View Previous Versions".)  Find the kit(s).
Download the kit(s) you want.  Read the "Release Notes" file for
instructions.