For those still having problems with connecting online with Resistance 2, especially those that uses routers and receives the NAT type 2, I have found a simple solution. First you should have access to your router's webpage; since I use a D-Link router, I can access it through http://192.168.0.1/. I'm not sure about other routers though. Then I go to the port forwarding part and add these ports here:
- TCP Ports: 80, 443, 5223, and 10070 - 10080
- UDP Ports: 3478, 3479, 3658, and 10070
Then after adding those ports (be careful with the TCP/UDP, you should really place those numbers where they belong) you should still turn on your UPnP (Universal Plug n Play) option in your router.
After fiddling with these configurations I can now join games in Resistance 2 online instead of the "Failed to connect" message I always get in the past. I hope this helps.