Nighthawk AC1900 won't connect through bridged modem- HELP!?!?!?

> I put the Westell into bridge mode- AC1900 as an access point. But in
> AP mode, I couldn't do port forwarding (advanced features were greyed
> out).

   That arrangement makes little sense.  You want a (NAT) router
somewhere.  But you don't want two of them.  Yes, when an R7000 is
configured as a WAP, many of its router features are disabled.  So,
don't do that.

> To avoid router address conflicts, Netgear setup assigned
> 10.0.x.x... [...]

   When an R7000 is configured as a router, and its WAN/Internet port is
connected to a router which is using the "192.168.1.*" subnet, then it
will choose a different subnet for its own LAN, typically "10.0.0.*".
That's one of the reasons for not cascading multiple routers.  So, don't
do that.

> [...] but the 192.168.x.x remained for the security system, and
> changing the Nighthawk back to a router mode caused a loss of
> connectivity.

   Your (unspecified) "the security system", and your computer, and all
your other client devices) should be configured to agree with the router
to which they're connected.

   There's no reason to hide any "192.168.x.y" address, or any
"10.x.y.z" address.  They're all private, hence useless to any hostile
outsiders.

> Got into the 6100 settings, changed it to 'bridge,' and it said router
> was rebooting, and completely froze.

   Define "froze".  I know approximately nothing about the 6100G, but I
would not be amazed if it got very stupid when put into modem-only
("bridge") mode.  At that point, the gizmo which is connected to the
6100G LAN port needs to do all the ISP-related work (PPPoE, or
whatever).

   If you want to use the R7000 as your (one, only) router, then the
6100G can't also be a router, and the R7000 must be a router, not a WAP.

> PC restarted, logged into westell menu screen. [...]

   How, exactly, did you do that?  What was the computer's IP address?

> [...] I selected 'restore defaults." This restored my internet
> connection but only as a standalone modem. [...]

   If "defaults" means modem+router, then that's not what you want.
What, exactly, does "standalone modem" mean to you?  Modem+router?
Again, that's not the mode which will let you use the R7000 as your
(one, only) router.  If it gives you Internet access that way, then at
least you know that its DSL configuration is ok.

> Verizon had always been a PPPoE connection...but what I've heard is
> that they've moved over to DHCP in the FIOS era.

   Is Verizon your current ISP?  If not, then who is?  I don't know what
you heard about what from whom, but I'd bet that your ISP can tell you
the modem/router configuration details for your/their service.

> I now have DHCP enabled [...]

   What you have is the 6100G configured as a modem+router, which is
(still) not the mode which will let you use the R7000 as your (one,
only) router.

   When configured as a modem-only, the 6100G won't do that.  But, while
you're there, if the thing is giving you Internet access, then you might
scout around on the 6100G interface and see if you can glean any useful
information on its DSL configuration.

   Lacking any useful (DSL-related) details, I'd do the following:

   1. Get the 6100G configured as a modem+router, so that you have
Internet access with it in that mode.

   2. With the R7000 connected to nothing but its power adapter, reset
its settings.  (Press its Reset button until its LEDs do interesting
things.)  After the settings reset (stable LEDs), power it off.

   3. Put the 6100G into modem-only mode.  Move the computer connection
from the 6100G (Ethernet) to the R7000 (LAN).  Connect the 6100G
(Ethernet) to the R7000 (WAN/Internet).

   4. Power on the R7000, and see if you can get a web browser on the
computer to talk to its management web site ("routerlogin.net",
"192.168.1.1", or whatever).  If the 6100G is in modem-only mode, then
I'd expect the R7000 (LAN) to be at "192.168.1.1", and your computer to
be at some other address on that subnet ("192.168.1.x").

   At that point, I'd expect the R7000 set-up wizard to try to explore
its environment, and discover how to configure itself accordingly.  If
it tries PPPoE, then you might need to supply any required PPP
credentials.

   If you haven't already, visit http://netgear.com/support , put in
your model number, and look for Documentation.  Get the User Manual (at
least).  Read.