How to tweak QoS settings in R6120

> I cannot find the QoS or anything related to that on my router
> settings page. [...]

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual (at least).  Read.  Look for
"QoS" or "Quality".

   If a feature can't be found on the product, and it can't be found in
the product documentation, then I'd take the hint.

   The R6120 is a bottom-of-the-line router; it's missing many things.