What brand of Modem - router do you have? Whilst LW's UPnP works well on modems that utilise that technology, not all modems that have internal firewalls such as NAT have Universal Plug and Play ability. Thus might require a port to be forwarded (opened.) Just a thought.

One way to check would be to connect via dial up if your isp provider has that type of backup service. I presume you have an internal modem. This would by-pass the other modem. If your incoming was true after this then you know the issue is with your usual modem. But you should do the bug test after LW's been open for at least 20 to 30 mins.

By the way could I ask who your isp provider is?