I don't know the content or duration of testing they and MS have done. All I now is that D-Link and MS have and are currently working with this router and I presume others to ensure both are working well using default settings. They didn't do any PFg or didn't seem to need it and only suggested PFg if default settings didn't work along with the other suggestions. In most cases, if the router is the only NAT, there isn't any need for PFg for XBL, at least with the 360. I presume the XB1 shouldn't either however anything is possible. If uPnP is enabled, this should handle most of the port behavior with connected devices with out PFg. I only recommend using the QoS on those routers that have that feature to handle the gaming and general traffic priorities if other devices are online while gaming.
NAT is a big issue and I've strived to help users understand that ISP modems can impact the use of external routers. I know you have a stand alone ISP modem, however some people don't so it makes it harder to configure routers to work with these modems with built in routers. Like my friend in CA, we got him 2 DIR-868Ls, one main router, the other in bridge mode for gaming. His ISP modem is a Motorola SB 6580 with built in router. We didn't get time to attempt to bridge it so we used the DMZ for the main host router. After going thru everything we got open NAT on his 360 and XB1. Ya, instant ON was disabled. We hope it keeps working. It sure is a process.
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Usually the process for setting up configurations on these routers are to start with reserving IPs. Then setting up the features that may be depended upon the IPs being reserved first.
Do you know how much testing they've done? As in, leave it hooked up for days and power down and up periodically to check the NAT. When I hooked up my Xbox One initially, it was reporting an open NAT. It wasn't until I checked again later that it had changed to strict.
On this topic, I did have a similar experience, not sure if it will help the OP or others. When I went to set up port forwarding, I did it in this order: set up the forwarded ports, saved, then set up the IP reservations for the Xbox One and saved. This caused the router to stop working. Even after a reset. I had to remove the port forwarding and IP reservation to get the router working again. I then did them in the opposite order: reserve IP, save, set up port forwarding, save. And that worked.