Some applications from internal network are required to be exposed to external Internet mostly for receiving connections. However in DD-WRT, settings are so confusing that it will not function immediately if you decide to use port forwarding.
A port forwarding rule is triggered when a request from somewhere on the Internet reaches the router so under its tab Source Net should be 0.0.0.0/0 or other specific ip/mask. The request is then translated and forwarded to a server behind a router so IP Address should look like 192.168.1.136 as an internal one. The rest options such as Protocol and Port can be easily configured.