See if your cable modem has a pass-through mode. This disables the NAT, DHCP, etc and passes the IP address of the cable side of the modem through to one of the ethernet jacks. Then you plug your own router in to that and do your own NAT, DHCP, firewall, etc. I know Arris used to have this, haven't checked recently.

My current ISP has the cable modem completely locked down. No control over DHCP, no punching service ports through the NAT for home servers. Also they've got me double NATed.