Back to the connections issue. If your OSX firewall is turned off & then you should not be seeing a brick wall. That's if your Linksys UPnP is working properly. BTW in UPnP mode, the router may decide to change the listening port itself. THe other option I can think of is port forwarding.

Is it possible to by-pass the linksys device ... do you have any other connection devices? Just to check whether Linksys is the issue here.

How old is your Linksys device? Have you checked for firmware upgrades?