RAX35 UNABLE to Connect to Router Setup

> [...] So I am thinking mind must be v1 by default.

   Yup.

> My intent was to use the AT&T Gateway as both a modem and router (NOT
> Wi-Fi) for my PC and route internet to my Netgear Nighthawk RAX35 Router
> strickly for Wi-Fi (as RAX35 seams to provide better wireless coverage
> inside and outside home than using the AT&T Gateway for Wi-Fi).

   Ok. In that case (connecting client devices to both the 5031NV and
the RAX35), it would make sense to configure the RAX35 as a WAP.  That
should put all your client devices on one LAN subnet, determined by the
5031NV, apparently "198.168.1.*".

   It's possible that when you first connected the RAX35 to the 5031NV,
the RAX35 set-up wizard decided on its own to do that.  Since then,
something could have happened to put the RAX35 back into its default
full-function-router mode, which would account for those "10.0.0.x" IP
addresses.

> [...] to the best of my knowledge there were no buttons pushed on
> either the AT&T Gateway, nor the RAX35 router, no outages, no setup
> routines ran. [...]

   Hard to tell from here, but regardless how it got this way, I claim
that you want to configure the RAX35 as a WAP.  (Which it seems not to
be now.)

> [...] Accessed the AT&T Gateway setup / maintenance page using the IP
> address [...]

   What was that IP address?

> Netgear states to use webpages WWW.ROUTERLOGIN.NET or .COM but they
> don't work... [...]

   They normally do, but only from the LAN side of the RAX35.

> '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" (or ".com") should
point to your own router, but that's true only if you use your own
router for DNS name resolution.  (Or, if the Netgear router can
intercept your DNS queries.)  Regardless, you should be able to get to
the thing using its IP address, if you can determine (or guess) that.

   Normally, that would be something like "192.168.1.1" (for a plain
router) or "192.168.0.1" (for a modem+router), but it could be
different.  ("10.0.0.1" is a typical alternate, for example.)

   A new complication arises if your web browser uses "DNS over HTTPS"
(and some real-world DNS server directly).  In that case, those
"routerlogin" names will typically take you to that misleading/useless
Netgear error page.  Specifying the IP address (instead of a
"routerlogin" name) should work, but there are some exotic cases where
you might need to disable "DNS over HTTPS" in your browser.

   In your case, with a computer which is connected directly to your
non-Netgear modem+router, those "routerlogin" names will take you to
that misleading/useless error page.

   I would:

   Disconnect the RAX35 from everything except its power adapter, and do
a settings reset.  When it recovers, connect a computer to it.  Ideally,
using Ethernet, but a wireless connection should work, too.

   Your computer should get an IP address like "192.168.1.x" (x = 2?),
and the RAX35 (LAN interface) should be at "192.168.1.1".  Use that
address or one of the "routerlogin" names in your web browser.

   Fight your way past the set-up wizard, and, at your earliest
opportunity, do that "Set up the router as a WiFi access point" thing.
That should make the RAX35 too stupid to be used on its own, so you'll
need to connect its WAN/Internet port to a LAN Ethernet port on your
main router (5031NV).

   At that point, you should be able to connect a computer to either the
RAX35 or the 5031NV, and get normal operation.

   The RAX35-as-WAP will, by default, get a new LAN IP address for
itself from your main router (DHCP server), using DHCP.  You'll need to
ask the 5031NV about its "connected devices" or "DHCP clients", or some
such thing, to learn which address it gave to the RAX35-as-WAP.

   When you know that address, you should be able to point a web browser
at it, from anyplace on your LAN, and get to the management web site of
the RAX35-as-WAP.  For greater convenience, you should reserve some
memorable IP address for the RAX35-as-WAP on your main router (DHCP
server), so that you'll be able to find the RAX35-as-WAP without slways
having to ask the main router where it is today.  How you do that on an
5031NV, I don't know.

   Unless you wanted to use the default wireless SSID(s) and passphrase
on the RAX35, you'll want to configure those as you like.  When you get
all its settings right, save them.  Look for "Manage the router
configuration file" in the User Manual.

   What could go wrong?