unusual setup with wndr3400 v3's

> [...] i have a spectrum modem, [...]

   Not a very detailed description.  Simple modem, or combination
modem+router?  A maker and model number could be helpful.

> [...] from router #1 i use a cat5 cable to router #2. [...]

   What, exactly, did you connect to what, exactly?  (Hint: If a device
has different types of ports, then "connected to device" is not enough
detail.)

> [...] i can access #1 [...]

   "access" _how_?  From what/where?

   Have you looked at the IP addresses of computers/devices which are
connected to any of this stuff?

> [...] #2 does not appear in the management screen for #1. [...]

   "the management screen"?  Do you mean an Attached Devices report, or
what, exactly?

> Cascading routers causes a double nat unless setup properly. [...]

   Generally, you want one router, not more.  Any other router-like
gizmo should be configured as a wireless access point (WAP), not as a
full-function router.

   This is what "properly" means.

   If your (unspecified) "a spectrum modem" is really a modem+router,
then you could use that as your one router.  That would allow you to
connect multiple (wired or wireless) devices to it.

   If your (unspecified) "a spectrum modem" is really only a modem (or
you can, and would like to, configure it as only a modem ("bridge
mode")), then you can connect it (a LAN port) to (the WAN/Internet port
of) WNDR3400v3#1, and use WNDR3400v3#1 as your one router.

   Any WNDR3400v3 which is not your one router should be configured as a
WAP.

   Visit http://netgear.com/support , put in your model number,
and look for Documentation.  Get the User Manual (at least).  Read.
Look for "Advanced Wireless Settings" and "AP Mode".

   If you configure a WNDR3400v3 as a WAP, then it would (by default)
get a new/different LAN IP address from (the DHCP server in) your one
router.  To determine that new/different address, you'd want to look at
an Attached Devices (or similar) report on your one router.

   It would probably be wise to use Address Reservation (or equivalent)
on your one router (DHCP server) to reserve a memorable address for each
of your WAPs, so that you won't need to consult your one router again,

every time you restart something.

   If you do that, then all your client devices should be on one big
LAN, with all IP addresses on the same subnet, and everyone should be
able to talk to everyone else.

   What could go wrong?