Unfortunately not all Linksys devices are good! The 54 range & especially the 54G range have firmware problems which Linksys never fully fixed. The effects are unreliable connection when run in both wireless & UPnP mode.

Probably first thing to try is deleting your LW preferences folder. Check various options here If LW 5 Will Not Connect

Best thing is usually to port forward the device for those models. The default port of 6346 is not necessary & can choose any port but best to choose are in this range 49152 - 65535. But make sure you do NOT choose same port as any other program uses. Router will reserve one port per program per computer. Set up Static ip address first is mandatory step otherwise the process will not work. PortForward.com - Port Forwarding Guides Listed by Manufacturer and Model & LW settings for LW 5 here: Manual port forward instructions & sample image