DNS and Streaming Device Issues with CenturyLink Fiber through my R8000P

> [...] I have a C4000XG modem/router combo from CenturyLink which I
> have disabled the wifi radios. [...]

   Disabling the radios in a modem+router does not disable its router.
(I'd expect disabling its router to disable its radios, but that'd be a
different case.)

   Cascading multiple routers can cause multiple problems.

> I connect the CenturyLink modem to the Nighthawk with an Ethernet
> cord.

   There's more than one Ethernet hole on an R8000P.

   Why are you using the R8000P with the C4000XG?  If your goal is
better wireless-network coverage, then you might want to configure the
R8000P as a wireless access point.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for "Use the
Router as a WiFi Access Point"

> 1) I can't use Google DNS or OpenDNS on any device or even the router.
> Once I try to change the DNS to anything but CenturyLink, I lose
> connection.

   Can't see what you're doing to what, where.  Don't know what "lose
connection" means to you.  I wouldn't expect DNS to be your biggest
problem with multiple routers, but I would expect some problems.

> When I switch my Nighthawk back over to my Cox cable modem, everything
> just works.

   Don't know what your "my Cox cable modem" is, but if it really is a
simple "modem", then that would be a significanly different situation
from using a modem+router with an R8000P-as-router.