Is LAN gaming possible?

I've recently changed out my Xfinity modem to the new Xfi and now my girlfriend and I can't LAN game anymore with ANY game. I've tried port forwarding and even connecting through our public IP with the port of the game we are tring to LAN but still we get nothing.

Is this new modem just not capable of LAN gaming? The current game we are trying to set up is minecraft.  There is a preset setting for it for port forwarding. I can't imagine it being this difficult. I've never had any problems just connecting LAN on my previous xfinity modem.