Can't get nat type on xbox open

[1337rofl]

So I've been trying to change my nat type from strict to open for a whole day now, but I can't seem to do it. I tried forwarding all neccessary ports in my router, and then assigning them to my Xbox one. After opening the ports, port checker still says they're closed for some reason.
I tried enabling Upnp, didn't change anything either.
Also I tried giving my Xbox one a static IP (assigned public IP to xbox one in router configuration).

And yet it says that my nat type is strict. Help please?

 

[PsyKhiqZero]

Just about every router has a DMZ function. Fine the xb1s local IP and put that address in the DMZ.

 

[1337rofl]

Just about every router has a DMZ function. Fine the xb1s local IP and put that address in the DMZ.

There's no option to type an ip in the dmz in my router configuration, but there is something similar: assign the public ip of a connection to a LAN device. I did this and selected the xbox one as LAN device, but my nat type is still strict.
If it helps, my router is a Technicolor TG789bvn