Port Forward by IP address not the nebulous "device" category

For various reasons I have a device with a single device / mac address and two IP addresses attached to it, 10.0.0.19 & 10.0.0.20. For reasons I need to forward a few ports on those separate IPs.
Unfortunately the Xfinity app seems incapable of doing this with the Xfi gateway. When port forwarding I only have the option of selecting a device and it only has the 10.0.0.19 address while the device details in the app only lists 10.0.0.20 instead of both.

When I select to add a port forward in the app how do I manually enter the IP to forward instead of the device from a drop down list.