Games will often require more than one port to be opened. I don't know if your modem is UPnP capable or not (or perhaps its UPnP implementation is not reliable.) So if you're having problems with your games also, manual port forwarding is the way to go. You'll need to find out which ports the games use. They often use more than one port.

LW only uses one port. By default this is port 6346. So I would recommend manual port forwarding for LW.

The steps: 1. Set up a Static ip (most important part & must be done 1st.) 2. Open a port. Instructions for steps 1 & 2 for your model modem is HERE (click on link). 3. Step 3 is also described there. Within LW set both the listening & connection ports to port 6346. Change setting to Manual Port forwarding. Press apply after changes. This setting is found here: LW's menu bar, Tools>Options>Advanced>Firewalls.
Good luck!