Switching to 2.4 GHZ on a dual band

> Does anyone know if I can set the R6220 dual band router to 2.4 GHZ
> only? [...]

   People who have read the R6220 User Manual should know.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for "Control the
Wireless Radios".

   Now, back to the real world...

> [...] I need a separate network using 2.4 GHZ only and google will not
> allow that. [...]

   Why, exactly, do you "need" that?  (And what, exactly, will "google"
"not allow"?)

> [...] I'm trying to set up a Ambient Weather STation and it will only
> work on a 2.4 GHZ network.

   So?  What's stopping you?  Do you believe that having both 2.4GHz and
5GHz radios working will cause trouble for a 2.4GHz-only device?

   Does the existence of FM radio stations stop an old AM-only radio
from working?

   General advice: You might have more success if you described the
actual problem which you are trying to solve, rather than asking how to
implement some particular "solution", which may have little or nothing
to do with the actual problem.