Open NAT

I have an Electrahealth Edge Router. I recently changed internet providers. The main difference between the two, is that my service now has a Public IP address. The Edge rounter has not been changed.

The issue is, that there are a couple games my kids play, (Call of Duty: Modern warfare 2, for example), that the xbox no longer connects to the game server. Message is: Failed to begin networking.

Researching this error, suggests that the NAT type needs to change from Moderate to Open. We seen how to do it on the xbox itself. I have two questions for you.

1. How can I change the NAT type to OPEN on the Edge router?

2. Is there a security concern if I do that?