Birdy suggested as far as she knows, that modem does not have UPnP support. They do have a Webroot Security & chances are this is the problem. ie: Compare Internet Security, & Antivirus Software from Webroot, and How do I use the Webroot Firewall to manage application traffic? (Opening for specific programs). This latter link appears to be the one to use to open the firewall for specific programs such as LW.

And more about Windows 7 Firewall here Understanding Windows Firewall settings in support of my link in my previous post #40 above.