Welcome to progress after the merger - Spectrum's internet includes the combo gateway unit (a modem + router) at no extra charge. Using WiFi from their box carries an extra monthly fee of $5 or so. Or if you choose to use your own router (as suggested by many experienced contributors to this forum), they can set their box to bridging mode, which makes it look like just a modem. Use a CAT-5 cable from the LAN1 port into your router's WAN port; then you manage your home LAN settings.
Remember, if you buy a cable modem and then switch to a different provider running over fiber, you throw away the cable modem, but an ethernet router can be reused.