What is double NAT? Basically double NAT happens when having 2 or more devices on your network (router or gateway) with NAT (Network Address Translation). When this happens traffic coming from the Internet may not properly reach devices inside the 2nd router thus causing gaming issues or problems with some applications and software programs. The most common scenario is when your Internet provider supplies you with a modem/router combo, and then you purchase a router and add it to your network.

How do I know if I have double NAT and how do I fix it?

There are a few ways to fix this:

1. Make sure your modem/router is it Bridge mode. A lot of modems supplied by your ISP may be a router combo. Most will have an option to put into Bridge mode (turn off NAT). Consult the manual for your modem/router or contact your ISP.

2. See if your secondary router supports Access Point (AP) mode. This will turn off NAT/router functions and act as a switch (for Ethernet devices) and access point (for WI-FI devices). The modem/router from your ISP will handle the router functionality. 

3. Enable DMZ in your modem/router combo. This will open and forward all ports to your secondary router. 

4. Bypass the WAN/Internet port on the secondary router and connect the modem/router to a LAN port. You will need to turn off DHCP on the router and statically assign the LAN/Local IP address of the secondary router to an IP on the same subnet at the modem/router. 

If you are having issues with multiplayer gaming, visit https://support.xbox.com/en-US/xbox-one/networking/nat-error-solution

More info - https://www.windowscentral.com/fix-xbox-one-double-nat