Security Camera FTP to NAS Through NetGear N6900 is intermittent

> [...] Through NetGear N6900 [...]

   Look for "Model" on the product label.  Firmware version?  Connected
to what?

> [...] The cameras and NAS are on the same LAN. [...]

   In that case, about the only participation by the router would be as
the DHCP server and, perhaps, as the network switch which connects your
(unspecified) "several wired security cameras" and your (unspecified)
"my NAS device".

> [...] When testing the FTP from the camera web pages, the FTP port 21
> fails in most cases [...]

   "testing" how?  "fails" how?

> [...] I suspect something has changed in the router firmware.. [...]

   Like what?  Why not Russian hackers?  Why blame the router, which is
barely involved in a LAN-LAN FTP file transfer?  You could reload
firmware (older, same, newer), reset the settings, and reconfigure it
manually, but I wouldn't expect any of that to change anything like
this.

> [...] Any ideas as to what may be causing this intermittent problem
> are appreciated.

   Knowing almost exactly nothing about any of these devices, I'd look
for better evidence of something/anything more specific before
condemning any particular device or speculating on a cause which could
be elsewhere.  A bad Ethernet cable some place seems about as likely to
me as any likely router problem, but I also don't know anything about
the wiring and/or any other network devices which might be involved.

   I'd also try doing an FTP transfer to your (unspecified) "my NAS
device" from something other than one of your (unspecified) "several
wired security cameras".

   And, when the problem recurs, I might look at the LED status
indicators on these mystery devices.