Can I connect a Nighthawk AC1900 cable modemrouter to a Blackhawk AX6 wifi router

> I bought the Nighthawk AC1900 cable modem router [...]

   "AC1900" is a speed, not a model number.  Look for "Model" on the
product label.

> [...] and the Blackhawk AX6 wifi router [...]

   Also not a model number, and not an accurate transcription, either.

> [...] I think I should have purchased just a modem and not a modem
> router - but I goofed. [...]

   Sounds likely.  One modem is generally enough.  If you can't swap the
Cxxxx modem+router for a suitable CMxxxx (or other) modem-only, then you
should be able to configure the Cxxxx as a modem only.

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Documentation.  Get the User Manual (at least).  Read.
Look for a topic like "Cable Your Modem Router to a Router and Use
Bridge Mode".

   An alternative would be to leave the Cxxxx as a modem+router, but
configure the "Blackhawk AX6" as a wireless access point.  But its
router is probably classier that that in the Cxxxx modem+router, so the
Cxxxx-as-modem-only is probably a better choice.