To open the NAT on the network, you need to change some settings of your router.
If your Internet service provider is a DSL company then make sure that your modem is in the bridge mode.

Basically there are some advanced settings on your router that you can change and set the NAT type to open.

Change the MTU settings on the router.
Reduce the MTU size to 1365. On some routers, you will not find MTU settings

Open ports on the router.
You will need to open the ports on the router according to the gaming console.
Click here to see the list of ports for different application.

Disable the firewall on the router.
If necessary then upgrade the firmware of the router.
Click here to know how to upgrade the firmware of the router.

To change the settings on the router, you will need to access the router set up page.
Click here to know how to access the router set up page