I'm a recent xfi 1gb user (got the right gateway after being mislead that my 400Mb/s bandwidth was all I would get for a 1.5yrs) and was trying to add a port forward through xfinity xfi like those above but it keeps adding the wrong ip address.
When I look at the device information in xfinity xfi web portal it shows the correct ip address, but when I select that device during the port forward it creates a port forward for an ip address I do not see in use in my gateway's admin interface. I already reset the gateway (not to defaults) but the issue remains.
I would have expected a way to input ip addresses manually in the xfi web portal by now.
It's not ideal since it would open all ports to your device, but you could enable DMZ which will allow you to enter the IP address of your device. If it's a PC, you could make sure that the firewall is enabled and block any ports/services that you need blocked.
@kidro2 some other users found both a workaround using reserved DHCP addresses or forgetting the device with the incorrect IP address.