I have a few game consoles running on static IP addresses.
I found this old thread detailing how to set a DMZ for a specific IP:
This guide used to work for me. Both my consoles got unrestricted internet access. However, since updating to the newest TO firmware, the external port mentioned in the link can no longer be left empty. And setting the external port to 0-65535 does not work. How do I set up a DMZ on the latest firmware?
If you use DMZ you can apply it for one console only and it is not good security. The correct solution for consoles is port-forward.
You have two choices:
install upnp and leave it to take care of it
set up manual port forwarding - in case of xbox it is port 3074 /etc/config/firewall > insert rule:
Then you can confirm it works, on Xbox go to Settings > General > Network > on right side you should read Current Network Status: NAT Type: Open. Furthermore you can run Test Multiplayer and Test NAT Type.
Also, the default port 3074 can be changed in xbox network setting, so that you can use multiple consoles and each one have its own different port forwarded.