Cisco DPC3941B and port forwarding

Hello,

We just got Comcast business service setup and are in need to setup some port forwarding. However, the public and private ports need to be different - i.e. hitting my external.ip:48483 should route to an internal.ip:3333. 

Unfortunately the UI only allows to setup a port forward range - and that obviously doesn't quite cut it.

Can you please assist in figuring out how to setup port forwarding on the DPC3941B with varying private and public ports?

Thanks!