UPnP works by the program communicating with the device to open ports. UPnP is relatively new & some brands implementation of it isn't good & is unreliable.

Manual port forwarding means that the port(s) are set to be opened within the device by settings within the actual device software. The port is set to the specific ip address. Static ip is required or else, if you have a dynamic ip, the next time your ip address changed the port setting wouldn't work. That's why a static ip is so important. Port forwarding is more reliable than UPnP.

Games & video conferencing have traditionally required many ports to be opened. LW only needs one port to be opened.