> Setting up a new Orbi with my old C3700 modem. [...]
C3700 or C3700v2? Look for "Model" on the product label.
By default, a C3700 is a modem+router, not a simple "modem".
Visit http://netgear.com/support , put in your model number, and look
for Documentation. Get the User Manual. Read. Look for "Change the
Router Mode".
> [...] The set up app says that the modem is not supported. [...]
Which "The set up app"? Use a web browser instead of the app?
Are you trying to set up the C3700[vX], or the Orbi, or what? For
the Orbi:
Visit http://netgear.com/support , put in your model number, and look
for Documentation. Get the User Manual. Read. Look for "Use a web
browser to access the router".
> [...] There must be a way?
There's a secret key:
https://en.wikipedia.org/wiki/RTFM
