Turn off WMM

> [...] I tried to use the user manual for N450, but there is no WMM in
> the menu. [...]

   Which "the user manual"?  Which "the menu"?  Visit
http://netgear.com/support , put in your model number, and look for
Documentation.  Get the User Manual.

   If there is no Setup > Wi-Fi Multimedia option (as described in the
User Manual), then firmware updates may have changed the options.  Can
you find anything about "Quality of Service" or "QoS", instead of "WMM"?

   A cable-TV ISP has complete control over the firmware of any
modem(+router) on its network.  It's also possible that your ISP has
disabled such features.

> [...] I see suttering when I use Airplay between my Mac and Apple TV.

   Do both these devices use wireless connections to the modem+router?

> [...] I have read [...]

   I've read things, too, some of which were more useful than others.