Can't ping WNDR3700V3 router connected to Comcast wireless router/cable from PC's on wireless router

> I have a WNDR3700V3 router hardwired [...]

   How, exactly?  (Hint: If a device has different types of Ethernet
ports, then "connected to device" is not enough detail.)

> [...] to a Comcast wireless router/cable modem. [...]

   Maker?  Model number?

> [...] I can't ping [...]

   "can't"?  Are the required keys missing from your keyboard, or what?

   "can't" 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 (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

> [...] the WNDR3700V3 itself or any PC's hardwaired to it. [...]

   Copy+paste of the actual commands and their results would obviate the
next question.  What are the IP addresses of these devices?

   If you cascade multiple routers as you seem to describe, then each
one might establish its own LAN subnet, which impedes simple
communication.  Most likely, you'd be happier if you configured the
WNDR3700v3 as a wireless access point.

   Many newer Netgear router models have a one-step WAP-mode option
which makes that relatively easy.  As I read the WNDR3700V3 Setup
Manual, it lacks such an option.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the Setup Manual (at least).  Read.  Look for
"access point" or "ap mode".  Prepare for disappointment.

   That's not fatal, however.  For an ad hoc scheme to deal with such
models, see, for example:

      https://community.netgear.com/t5/x/x/m-p/1463500

That's written for a Netgear C6300-as-WAP, but the steps are about the
same for any other router (any make/model) which lacks a one-step WAP
option.  If the router which you're reconfiguring has a WAN/Internet
Ethernet port (unlike a Cxxxx), then leave it unconnected.