Help with forwarding to internal port?

I'm running Victek 1.3g for my AC66U.

I am having trouble forwarding an external port (say) 7777 to an internal port 554 at address 192.168.2.120.

If I just simply forward external port 554 to address 192.168.2.120, things work as expected.

I am trying to do this via GUI....can someone help me?