Cannot configure new Nighthawk with web browser

> Model: R7000|AC1900 Smart WIFI Router

   Firmware version?  Connected to what?

> I am unable to access the web interface and can't configure it. [...]

   "unable" 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.

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the User Manual.  Read.  Look for "Set Up the
Router as a WiFi Access Point".  Pay particular attention to the
prescribed wiring scheme.

> [...] I used the iPhone app for this, and can't reconfigure to AP mode
> from there.

      "Doctor, it hurts when I do this."

      "Don't do that."

   The management web-site interface on the R7000 exists for good
reasons.

> I need this to connect to my local LAN (via the Nighthawk LAN
> connector), leave the Nighthawk WAN port unconnected, [...]

   You might have a better chance if the R7000 was not connected to your
other stuff during its configuration as a WAP.  See link below.

   Regarding "to connect to my local LAN (via the Nighthawk LAN
connector)", see the User Manual section cited above.

> [...] I tried both the WAN and LAN connectors for this.

   "this"?  I would not expect that connecting a client device
(computer, ...) to the R7000 WAN/Internet port would be productive.

> [...] use a static IP of 192.168.99.6, disable DHCP, and use AP MODE.
> [...]

   That should be possible.  I'd suggest, however, using a dynamic
address reservation on your (unspecified) main router, rather than an
actual static address (configured on the R7000-as-WAP, itself).

Selecting "AP Mode" should disable the R7000 DHCP server without any
additional explicit action.

> [...] I cannot do this with the Nighthawk app.

   See "Doctor, [...]", above.

      https://community.netgear.com/t5/x/x/m-p/2003102

> I need a dual-band AP that supports about 6 wireless streaming clients
> with no problem and has great bandwidth.

> I've only seen rave reviews for this box; did I buy the wrong thing?

   I don't know if it'll be suitable for your situation, but configuring
it as a wireless access point should not be an insurmountable problem.