Spectrum suggest to use an authorized modem but if you are wondering, can I use my own router with Spectrum. Yes, you can. You just require an Ethernet cable to do so.
Steps to connect router and spectrum with Ethernet:
 Plug the Ethernet cable in the modem.
 The other end of the same cable needs to be plugged in your router.
 Switch on your router.
 It will take up to 3 minutes to start.
 Now, turn off your computer and connect the Ethernet cable to your computer.

Read more: change spectrum router settings