@RMerlin - Just FYI, I have tried use my Xbox One and 360 in the DMZ, so ALL ports are open, the result is that the NAT type is STRICT no matter what console IP I use.
Example, I have my Xbox One @ 0.6 - I put that in DMZ, comes back as Strict NAT, so I clear that and put my 360 IP of 0.7 and that reports a strict NAT, and before stating I am using a static IP for each that's all I ever use, so I assume DMZ doesn't work as it should as it has in the past, now I assume it might work for you, but it doesn't for me I have cleared NVRAM and started from scratch and still I get the same results, this also applies to UPNP not functioning as it should here is a list of methods I've tried..
1. Tried individual IPs in DMZ, no success.
2. Disabled DMZ, and just UPNP, no port triggering, no port forwarding enabled, no success.
The ONLY workaround that works for me is that by port forwarding the Xbox One, I get an OPEN NAT, then turning off and on UPNP to get my 360 to work as an OPEN NAT, which I have to do every time I want to be able to use my 360 correctly.
All other features of the router work, it's mainly those two problems, now I don't know if other members have had similar problems but I am making you aware of a problem that has arisen for me,