In theory port forwarding should work, provided that you followed all the steps correctly (not trying to make you feel bad, I stuffed mine up the first time). My modem doesn't support UPnP so I don't use it myself but... it sounds weird that yours should disconnect you from the net when you enable UPnP. Your modem has a thing called "bridge mode" that somehow enables it to become a router as well, which I don't understand at all It's possible that this could be causing probs??

Did you get your modem from your ISP when you signed up, or did you buy it yourself? If it was part of an internet plan, give your ISP a call & see if they can help you out. If they sold you the modem, they should be able to help you to configure it properly. I can't find any tech support site for Efficient-Siemens, have a look in the manual & see if there's a site/phone number listed. Sorry I can't help more!!

Edit - finally found this site, see if it's any help
http://subscriber.communications.sie.../support.shtml

Also this info about firewalls. Turning off the NAT firewall altogether's not a good idea but you may be able to customise it to allow access for Gnutella?? I'm really not sure.
http://www.alltel.net/downloads/links/firewall/