Can't make R7900 into AP!

> Got a XR500 to make my R7900 into an AP. [...]

   Eh?  Does that mean that you got an XR500 to use as a router, and you
have an old R7900 which you want to configure as a wireless access
point?  (Because getting an XR500, by itself, would not "make [your]
R7900 into an AP".)

> [...] following directly directions for setting up the R7900 as an AP,

   Which "directions [...]"?  Which choices did you make?

> [...] I can't log in to the 7900.

   When you do what, exactly?  "can't" is not a useful 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, LED indicators, ...) can be more helpful
than vague descriptions or interpretations.

> [...] Even after doing a reset to see if it helps, nothing. [...]

   "doing a reset" on what?  The XR500?  The R7900?  (Re: "nothing", see
"can't", above.)

> [...] What am I missing. [...]

   What, exactly, are you doing?

> [...] log into the AP again via http://www.routerlogin.net,
> http://www.routerlogin.com,

   I don't see how that could work.

> [...] or by typing the IP address of the AP into the browser.

   _That_ could work.  (_If_ you know that IP address.)

> This is misleading - the routerlogin.net or routerlogin.com can only
> work if you are connected (ie. wireless) to the Access Point. [...]

   Not even then, I'd say.  I'd expect those "routerlogin.{com|net}"
names to work only to access the router (acting as a DNS server), never
to access a WAP.

> [...] PC sees it in IE browser as me trying to login to the XR500.

  That's what I'd expect from any "routerlogin.{com|net}" names.  To
access the WAP, I'd expect to need to use its (new) LAN IP address.

   Given that, I'd avoid that KB article, and stick to the instructions
in the router User Manual.

   When an R7900 is put into WAP mode (ADVANCED > Advanced Setup >
Wireless AP), you decide whether to let the R7900-as-WAP get a LAN IP
address from your (router's) DHCP server ("Get dynamically from existing
router"), or to give it its LAN IP address yourself ("Enable fixed IP
settings on this device (not recommended)").  If you specify the
default, "Get dynamically [...]", then you'd need to extract its LAN IP
address from an "Attached Devices" (or "DHCP Clients", or similar)
report on the "existing router".

   For future convenience, you might want to assign a reserved address
for the WAP in the (router's) DHCP server.

   Alternatively, you could specify a (static) address "Enable fixed IP
settings [...]"). in which case, you'd know that address.  If you do
that, then you'll want to shrink the DHCP pool (on the XR500) from its
usual default (".2" - ".254") to something smaller (like, say, ".2" -
".199"), and then use an address from the non-pool range (".200" -
".254") for the R7900-as-WAP.

> [...] I'm still extremely aggravated from hrs of torture. [...]

   Faulty instructions can do that.