Can't access the Router Login page after doing factory reset.

> I am not able to access the routerlogin.net (or any other IP address
> inputted) page to set up my router/modem, no matter what I try.

   "I am not able" is not a useful problem description.  It does not say
what you did.  It does not say what happened when you did it.  As usual,
showing actual actions with their actual results (error messages, ...)
can be more helpful than vague descriptions or interpretations.  What,
for example, might "any other IP address" be?  Did you try all
four-billion-odd possibilities?

> [...] No matter what IP address or URL I put in a browser address bar,
> to access the router login page, [...]

   Some (any) actual examples would have much more value than any such
generalization.

> "You may not be connected to your Router's WiFi network. [...]

   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.  Perhaps:
"192.168.0.1".  You can often get it from your (unspecified) computer's
IP configuration as the default gateway or router address (or some
similar description).

> [...] also tried using my default gateway IP in addresss bar [...]

   What, exactly, was that?  If your "the PC" is running Windows, then
open a Command Prompt (CMD.EXE) window, and issue an "ipconfig" command.
Copy+paste the report here.