Is there anything you've changed in your system or settings that may have contributed in you losing connection?

I don't know whether the Airport supports UPnP. If it does, then make sure it's enabled. If not then you may need to manually port forward it. I think there's some instructions in the support section on that link you suggested you looked at. Connection problems? Check here first! However if you do this, you'll need to do a separate port forward for each computer & use different port no.'s
Make sure the port you've cleared in the firewall is the same as the one in your LW Prefs>Advanced>Firewall settings for both Listening port & manual port forward OR UPnP port. Make sure these are the same.