Can I turn off wifi on N600 WiFi Cable Modem Router?

> [...] We have coaxial connections in a new house, but only the one
> connected to the modem has been working. [...]

   "connections" between what and what?

   Define "working".  "not working" 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 first question here is whether the cables are "not working", or
whatever you're trying to do with them is goofy.

> [...] We will be getting the coaxial ports fixed [...]

   Let's assume, for the moment, that they need to be "fixed".

> [...] and getting cable modems to plug the coaxial cable into and then
> ethernet into the laptops soon [...]

   To me, that sounds like a very bad plan.  Multiple cable modems?  Ask
your ISP how well that will work.  (I wouldn't expect a favorable
answer.)

   What _would_ make some sense would be to put one cable modem+router
(like your C3700v2, or else a cable modem and a separate router)
someplace, and run _Ethernet_ cables (or equivalent) between that
(modem+)router and your computers.

   If running Ethernet cables would be very difficult, then there exist
adapters to use existing power wiring or co-axial cables, instead, but
these tend to cost more than plain-old Ethernet cables, and may be less
fast and reliable.  A Web search for, say:

      powerline adapter
and/or:
      moca adapter

should find many things.

> Is there a way to turn off the wifi and just use the wired
> connections?

   Sure.  Visit http://netgear.com/support , put in your model number,
and look for Documentation.  Get the User Manual.  Read.  Look for
"Control the WiFi Radios".  But many people wouldn't bother.

   And disabling the wireless-network radios wouldn't help in your goal
of connecting multiple computers/devices by Ethernet to anything.