Cant keep me NAT Type open on my xbox one s. I have AT&T fiber router.

You should contact AT&T for help with their router

If you use UPnP you don´t need Static IP´s. On my Xbox (only one) I use UPnP and if I give my XBOX an Static IP my NAT type gets back to moderate. That makes no sense for me - maybe it´s the implementation of UPnP in my router. Only if I don´t give my xbox
a static IP and use UPnP my XBOX works with open NAT. Try to change your xbox to a non static IP Adress. and restart your Router.

I have a NAT Setting on my router but it is set to on? When I have it on automatic my NAT type stays open. I don't know why DMZ won't keep my NAT type open.

Both of the 360 consoles are on a wifi (wireless) connection. Each one is connected to separate routers. All necessary ports are open and the NAT Type on my Xbox One is 'Open'. The Xbox one is connected to one of the routers with one of the 360's.

...but placing the Xbox One's IP in the DMZ in the router's settings gave me consistently open NAT.

^^^THIS.

Using the 'trick' Danielson mentioned will work... but from what I've been told it's only temporary and you'll have to keep checking to see if the NAT has reverted back to moderate
or strict. Seems kind of silly for them to even have that option... if it doesn't keep your NAT open... why bother?

I put my X1's IP in the DMZ and I always, always, always have open NAT. DMZ = Set it and forget it.