I followed the post that is here http://forums.dlink.com/index.php?topic=58975.0 and am still having problems.
I have 3 Xbox 360s each with an open NAT and one Xbox One with a strict NAT.
My ISP is Clear which is now Sprint, fastest I can currently get. I average 3 Mbps (6 peak) down and 1 Mbps up.
The modem is a Clear Series M (Motorola). This is directly wired to my DGL-5500 and the Xbox One is wired to this as is one of the Xbox 360s.
I have exchanged cables and tried turn off the "Instant On" feature of the Xbox One. This last one opened the NAT for a few minutes.
WPS is off and I have checked the MTU. MTU is default of 1500 works OK but ISP says to use 50. This seems low to me but it's what Clear has told me. I found by using FurryNutz technique a value of 1398 is best.
I have tried the other items that FurryNutz has stated in the post referenced above.
I do have two Port Forwarding applications setup: 1) the main Xbox 360 for Xbox Live that opens TCP 53, 80, 3074 and UDP 53, 88, 500, 3074, 3544, 4500 and 2) the Xbox One for Xbox Live that opens TCP 53, 80, 3074 and UDP 53, 88, 3074. I have turned these on and off in various combinations but doesn't fix the Strict NAT of the Xbox One.
I'm open to suggestions as I've been trying to figure this out by myself for a while by reading this forum.