setting up Orbi RBK13 with C3700 modem

> 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