Firmware is invalid.

> [...] Unzipped the file and uploaded the chk file from the zip. [...]

   It's normally hard to get a corrupt file (without a complaint) from a
zip archive, and even harder from multiple zip archives.  If the router
rejects Netgear firmware files and third-party firmware files, then it
seems more likely that the router's little brain has been scrambled than
that all the firmware files in the world are corrupt.

   If the web-interface load procedure fails, then you might try using
the (last-ditch) TFTP scheme, which might take a different enough path
through the software to give a different result.  It's also possible
that the router has a hardware problem (bad NVRAM).  I've never tried

it, so I know nothing, but:

      https://kb.netgear.com/22688/

> Really, I know how to [...]

   Apparently so.  Now.

   Few people who post questions here begin with anything like, "I made
the following errors..."  Assuming (without evidence) that anyone
actually knows anything usually leads to much wasted time.  In contrast,
providing accurate, detailed information normally saves everyone's time
and effort.

   Why did you decide to do this update?  Actual problem, general
principle, ...?