I'm all for customers using their own Wireless as a stand-alone router will almost always work better than the modem/router combos we provide. And its a lot easier than you may think. You could most likely go into a Best Buy, let them know you want a router for your 300mbps through Spectrum, and they can point you in the right direction and advise you of basic setup steps. And most routers these days come with easy to read, detailed instructions of getting setup in just a few minutes of connecting your router.
@CableAlan, trust what the Spectrum employee is telling you: The second (Internet) modem is included in your internet price. If your incoming signal is stronger than -6 dBmV, you just buy a Gigabit router with 3 or more ugly external WiFi antennas. Then tell Spectrum to turn off the WiFi in their original gateway, which should eliminate their $5 monthly WiFi fee.
You get to set up the router yourself to meet your networking needs. Spectrum cannot get into it and change any of your settings.