Modem with port forwarding setup

SBG8300, G34, G36 and G54 are the device compatible with Spectrum and its a combination of cable modem and router so it has port forwarding option on the modem GUI or web interface also on the surfboard central app. 

When you are connecting separte router ( Luxul)  to the modem you can check for the port forwarding option on the router interface. 

If you want to use the cable modem from Arris you can check for SB8200 or S33 device which doesn’t have any advanced setting so for port forwarding you need to check on the router.