I have set the MAC address to the HFC MAC address of the SBG6580, the same MAC address that Cox has registered to allow my device to operate on their network.
Why did you use MAC cloning? That will only mess things up. Try without it. Just make sure to power cycyle the gateway after hooking up the Netgear router.
In general, I would get a stand alone modem rather then a gateway. BTW, what model is the Netgear 2100? Sure you don't mean Netgate 2100? Or the Netgear AC2100?
::edit::
I am going to assume you mean the Netgate 2100 because that makes more sense and you say it more in your OP.
The whole thing boiled down to the SBG6580 being crippled through firmware pushed onto it by Cox. According to the docs and others that have tried the same thing on different ISPs, the device can be put in bridge mode and work as I wanted as long as the firmware still supports it. As Darkatt pointed out it is old equipment, so I replaced with a new device that only functions as a cable modem. Once I did that, the Netgate router worked properly with no more issues.
