R7800 - Nighthawk X4S AC2600 Smart WiFi Router

> [...] when I go the the routerlogin.net website, I get an error
> message that I am not connected to the router's wifi network. [...]

   If you mean that you got an error page which said, "You are not
connected to your Router's WiFi network. [...]", then welcome to what
may be the worst error message in the world.  For an explanation, try:

      https://community.netgear.com/t5/x/x/m-p/1365830

   Briefly, the idea is that "[www.]routerlogin.net" should point to
your own router, but that's true only if you use your own router for DNS
name resolution.  Irregardful, you should be able to get to the thing
using its IP address, if you can determine (or guess) that.

   The User Manual for your router should tell you what its default LAN
IP address is.  Look for "192.168.".  Otherwise, you can often get it
from your (unspecified) computer's IP configuration as the default
gateway or router address (or some similar description).  Visit
http://netgear.com/support , put in your model number, and look for
Documentation.

   You can use any DNS server(s) you want, but if you really want to use
some other DNS server(s), then you should not expect
"[www.]routerlogin.net" to work.  The router's IP address should always
work.