I have a Mac running OS X 10.7.2, and a TP-Link TL-WR740N (whose control panel looks like this). Remote Login is on in the Mac's System Preferences, and port 22 is set to forward on the router. I can access my Mac as a web server using the external IP on port 80, which I have set up through the same port-forwarding mechanism provided by the router, but when I try to ssh server@external-ip, it just times out after a long while. (The same thing happens when I try vnc.) I can, however, ssh and vnc successfully into that computer while I'm on the same network when using its internal IP.

Since ssh appears to work and port forwarding appears to work, I can't figure out what's causing the problem. Does anyone have any idea what might cause this?