Problem with using internet IP

[yetanotheruser]

I'm trying to set up SC but it seems to not like me using my public IP. I've set up my router to forward a port to my local IP, given myself a DHCP reservation for that IP, and checked that my local address, internet address, and port numbers are all correct and in agreement on everything I'm using. Here's what works and what doesn't:

Working:
Using SC configured with a local IP. Connects nearly immediately.
Port forward testing for both public IP and dynamic-DNS works and pops up a VNC listener error from unknown protocol. This tells me the IP address and port are correct and correctly forwarding.

Not working:
Using my public IP address or a dynamic-dns service address in SC. Sits on establishing connection, times out instead of establishing connection.

Any idea what's happening? Any idea what I might do to resolve this issue?