But still we are already getting somewhere

You say that Limewire worked before using the wireless router

Did you by any chance forget to add Limewire.exe or your set listeningport to your Windows Firewall ?

Have you upgraded your Limewire lately ?

Has the brick wall always been there ?

Also are you sure that your new router really supports UPnP (not just says it does)

Trying Portforwarding your Limewire port is an option. In which case anyone trying to help will need exact name and model of the Linksys