If you access your router through the web interface and go into the configuration settings you can set up a "Virtual Server".
Put the port number that you wish to open ie 6881 in the TCP part and the LAN ( Local Area Network) IP address of the pc that you are going to use for bit torrents.
Your LAN IP will probably be in the ranges of 10.x.x.x or 192.x.x.x , depending on how your router assigns the IPs.
You dont need to do anything with the WAN, (Wide Area Network) IP, that is the IP address that your ISP provides and will change each time that you reconnect as you said that it was Dynamic,
Some routers allow you to enter a range of ports, mine only allows individual ones so I have to repeat the steps for each port.