Have router set as DHCP and NAT and connecting with DHCP
Please make sure that you do not have a Double NAT error on your network, which will make port forwarding difficult to impossible to configure correctly.
To do that, open AirPort Utility on your Mac.....(Finder > Applications > Utilities > AirPort Utility)
Click on the picture of the TC, and look at the smaller window that appears
Make sure that there is NOT a Double NAT error next to the Status listing.
Also, please keep in mind that most "port checking" utilities are basically worthless. By that, I mean that when I have Port Mapping set up so that I can access my TC remotely, one of the requirements is that Port 548 must be "open". I just checked using three different popular utilities and found that each one was wrong....indicating that the port was closed when I know it is open, because I am currently connected back to the TC from a remote location as a test.