How to configure home network Router is a AC1600 Model R6260

> [...] I want to connect the 2 routers together, via the LAN ports,
> [...]

   Concentrate on specifying what you want to happen, not on how you
expect to implement the solution.  What, exactly, is connected to what,
exactly, now?  Hint: If a device has multiple Ethernet ports, then
"connected to device" is not enough detail.

   Visit http://netgear.com/support , put in your (actual) model number
("R6260"), and look for Documentation. Get the User Manual. Look for
"Use the Router as a WiFi Access Point Only".

> The piece of the puzzle I can't figure out is how to log in the aC1600
> without doing a factory reset. [...]

   When configured as a wireless access point, the R6260 will have a
non-default LAN IP address, either a dynamic address supplied by the
(DHCP server in the) main router ("Get dynamically from existing
router"), or else a static address which you specify ("Use fixed IP
Address (not recommended)").  In the latter case, you'd know which
address you gave it.  In the former case, you'd need to ask the main
router which address it supplied, typically available from some kind of
"Attached Devices" or "DHCP Clients" report.

> [...] With DHCP disabled on the AC1600. [...]

   It sounds as if you're using an ad hoc WAP configuration, rather than
the WAP-mode option built into the R6260 firmware.  (In that case, you
_would_ want to avoid the WAN/Internet port on the R6260.)

> [...] The IP address for the AC1600. has been changed to avoid
> conflict with the router connected to PC#1. [...]

   "changed" to _what_?  How/where, exactly, did you configure this
different IP address for the R6260?  You should be setting its LAN IP
address, not its WAN/Internet IP address.

> [...] Using the IP adress or routerlogin .com does not get me to the
> login page, I get a page stating I need to be connected to the router
> ( it is connected to the internet ).

   As usual, showing actual actions with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.  Copy+paste is your friend.

   If the actual error message said something like "You may not be
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

   That failure of a "routerlogin" name would be expected for an
environment where the main router is not a Netgear router.  I'd expect
the LAN IP address of the R6260 to get you to its management web site
from anywhere on your LAN, but some complications are possible.

   If your client device is connected to the R6260-as-WAP, then the
R6260-as-WAP may be able to intercept a "routerlogin" name, and handle
it itself, but, if your client device is connected to the main router,
then you can expect problems with any "routerlogin" name.

> [...] BTW I am out of my comfort zone at the minute. [...]

   That may change.  The critical thing about a WAP is that it's
all-LAN. (The special WAP mode option configures its WAN/Internet port
essentially as an extra LAN port.)  The router functions of the WAP are
disabled, so it acts as a simple network switch (with wireless
capabilities).