Attempt to add a Port forwarding rule on Cisco Technicolor DPC3941B fails every time

Hi,

Every attempt to add a Port forwarding rule in the Cisco Technicolor DPC3941B fails with the message:

I have read the how to on the support site:

https://business.comcast.com/help-and-support/internet/comcast-business-ip-gateway-static-firewall/

But the page above mentions fields in the portforwarding that the UI i see does not have: PUblic Port and Private Port. I have Start port and End Port. 

The Software version is:

Software Image Name:DPC3941B_3.9p6s3_PROD_sey

The server computer on the LAN has a static IP that is NOT in the DHCP range of the router. But when i go to the "Connected Devices" section, the computer is listed as "DHCP". It had been a DHCP config but then assigned it a static IP. So the router has the MAC address and may be confused. ANd this may not matter but thought i would provide the info in case it does.

When i select the server, i click the "Connected Device" button to select the server's IPV4 and IPV6 addresses and have them populate the fields.

I've configured a lot of routers. I'm either missing something, something is not as it seems, the router needs a firmware update, or maybe just a reboot.

Any help will be much appreciated.

Peace,

BAG