how to block devices from accessing local network without denying access to the internet

> My Q-See security cameras need to be hosted inorder to view via web.
> [...]

   "hosted"?  I don't know what that means.  How do you access your
(unspecified) "My Q-See security cameras" locally?  Web browser?  App?
Other?

> [...] As q-see has gone out of business it has recommended to use
> remote access via DMZ and enable DHCP. [...]

   It might be more helpful to see the actual recommendation, rather
than your interpretation of it.  ("enable DHCP"???)

> [...] I understand this basically turns off your router's firewall
> and allows an outside connection directly to your NVR. [...]

   There's some kind of (unspecified) "NVR" involved?

   Specifying a DMZ server is probably over-broad, but any scheme which
allows external access to one of your (unspecified) gizmos would "allow
an outside connection directly to your" gizmo.  Presumably, there'd be a
password involved for such access.

> How can I block this outside connection from all the other devices on
> my LAN?? [...]

   Don't specify a DMZ server.  Knowing nothing about your (unspecified)
gizmos, I'd guess that one or more port-forwarding rules would allow
external access to one or more specific devices (only).

> Model: R7900|Nighthawk X6 AC3000 Tri-Band WiFi Router

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for "Set Up Port
Forwarding to a Local Server".