For Netgear Nighthawk R7000 router only! (Maybe you can try the trick for another Netgear router but I have the R7000 and can confirm this works)
ISSUE: Two xboxs on the same network connection getting errors when trying to play Halo 5 Guardians together.
If you are like me you checked everything and every other game works perfectly except for Halo 5. You have open NAT, wired/wireless, UPNP etc, basically your connection is fine.
This issue is being caused by the routers latest firmware. Go here http://downloadcenter.netgear.com/en/product/R7000#searchResults
Download Firmware Version 1.0.3.24 (802.11ac Wi-Fi CERTIFIED)
You can try others, but I can confirm this one works.
Login to your router 192.168.1.1 or routerlogin.net
Advanced > Administration > Router Update > Browse > Find that folder you just downloaded > Select the .chk file and update router to old firmware.
Hard reset both xboxs, check your NAT and you should be able to play now.
NOTE: There are also reports of people having the same issue with their Arris Modem/Router provided by Comcast/TWC so if you have a Nighthawk R7000 and that modem/router you could have a problem. However if you have your Nighthawk and a standalone modem like a motorola surfboard this will work.