It might interest some of you to know that the latest xbox insider preview build contains new network features and improvements to address this issue. There's discussion about it on reddit started by one of the xbox engineers whom I quote below: https://www.reddit.com/r/xboxone/comments/6si38n/psa_some_small_but_interesting_networking_changes/
Alternate multiplayer port selection: Under Network settings, Advanced settings there's a new button labeled Alternate port selection. For home networks where UPnP is problematic or unavailable and where multiple Xbox consoles are present, customers can now select an alternate port for multiplayer and chat to use. The default port for Xbox multiplayer and chat is UDP 3074, and when working around NAT issues you can set up either port forwarding or port triggering for this port & protocol to open up the NAT type for a single console. However, this option is only available to one console at a time if all consoles are using UDP 3074. With this new option, you can select a different UDP port from a drop down menu for each console so that you can setup port forwarding rules for each console on your home network. When the console binds to the new port, it will also attempt to port map via UPnP on this new port in case the UPnP issues with the home router are intermittent.