Using a Nighthawk AC1900 exclusively as a router

> I have a Nighthawk AC1900 cable modem router [...]

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

> [...] Spectrum requires use of their cable modem. [...]

   Has _it_ a maker and model number?

> [...] I would like to use their cable modem and continue using the
> Nighthawk AC1900 router capabilities. Is this possible, [...]

   Assuming that your "a Nighthawk AC1900 cable modem router" lacks a
WAN/Internet Ethernet port, then no.

   What, exactly, does "router capabilities" mean to you?  It should be
possible to use a model Cxxxx as a wireless access point.  See, for
example:

      https://community.netgear.com/t5/x/x/m-p/1463500

That's written for a Netgear C6300-as-WAP, but the steps are about the
same for any other router (any make/model) which lacks a one-step WAP
option.  (Especially similar for another Netgear Cxxxx.)