N750 / WNDR 4000 unable to sign in

> I have no idea.  I didn't see or didn't take note of a version listed
> when I performed the upgrade. [...]

   Ah.  One of the advantages of manually downloading and applying an
update is that that scheme gives you a durable record of what you got.
If the easy/automatic scheme fails, it eats all the evidence.

   In the words of the emminent Abraham Van Helsing, M.D., D.Ph.,
D.Lit., etc., etc., "Remember, my friend, that knowledge is stronger
than memory, and we should not trust the weaker."

> I am able to access the internet, however.

   Yup.  Probably only the web server gets whacked.

   If the new firmware makes it impossible to reach/use the normal
Firmware Update page, then the only way to recover may be to use the
TFTP scheme to load a different (presumably older) firmware image:

      https://community.netgear.com/t5/x/x/m-p/1502275