I am sorry about the issues you are having with your Xbox. When you set it in passthrough mode, it should resolve the issue because it is no longer behind a NAT. In passthrough, it should receive the WAN ip address. Make sure you do not have anything else set in passthrough. Also, I always prefer to use DHCPS:fixed so you can input the MAC address of the device to ensure you are setting the right one up in passthrough mode. Check in your Xbox settings to see what IP addres you are getting. If it is 192.168.1.x, it is not in passthrough mode. Try to restart the devices to see if that will help.
Hope this helps.
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