For best performance please connect your Playstation 3 via Ethernet cable and then try again and let us know how it goes. But it should work fine, Try checking if you are out of range of wireless signal from the playstation 3 and Dgl4500. One other main issue can be your Isp maybe your internet connection is not fast enough to keep up with the host of the match. Because when you invite someone to your party and you go into a game it could most likely be is that the game matchmaking kicks you out because of bad connection to host. Did you try forwarding? If you did not i would be most happy to guide you through it. First login into the router interface, once in, Navigate to advance then gaming. Add a gaming rule.
ADD GAME RULE
Enable : check
Name : Call of duty Mw2
IP Address : Your static Ip you gave it. Fury nutz has link top ^^^^
TCP Ports to Open : 27014-27050
UDP Ports to Open : 1500,3005,3101
Inbound Filter : Allow all
Schedule : Always