Having trouble connecting an Xbox One to the Cable Modem Router C3700 - N600

I can connect the Xbox One through a wireless network with the router. The issue arrises when I try to connect the Xbox One to a ethernet cable from the back of the router (it needs to be wired because the xbox normally sits too far away from the router to be wireless). I've called the ISP and they have no idea what they're doing, but they insist that they're not the problem. I've had some tech-savy friends tell me I need to do port-forwarding on the router, which I've tried doing but that didn't seem to resolve the problem. The xbox keeps telling me that it is connected, but the NAT Type is Strict, so I can't use any of the internet features.

PS. Please, please, please do not tell me that the issue might be the Xbox One. I've connected this xbox to tons of networks (wired and wireless) and I've never had an issue like this before.