How to set up port forwarding to an IP address instead of a 'discovered' device.

I have several complaints about how dumbed down the user interface is for the settings on the CGN4141. The biggest problem is the port forwarding. I have a Cisco 1921 that sits behind the CGN4141 and I need to port forward to static NATs I have built on the Cisco 1921. Since this brain numbingly simplified device can only port forward based on MAC address, I can't accomplish what I need to. Anyone know how to get around this?

IE 1.1.1.1, 1.1.1.2, 1.1.1.3 share mac address 00:00:00:00:00:01, Cox only sees 00:00:00:00:00:01 and keeps changing the IP address.