Not being argumentative either. Just stating what I've done and some of what you have expressed has already been expressed as well higher up in this thread. Just saying that some of this has already been looked into.
When you had it working with one ISP service on two consoles and OPEN NAT across the board, what mfr and model router were you using?
There are a few Mfr routers that continually provide OPEN NAT across the board using FULL CONE NAT. There listed on my post on badmodems.com.
When I've had it working, it's been on the Asus RT AC-87. Both consoles wired straight to the router. I had it working the 1st day I installed the alpha, haven't had both Xbox's here to try it since.
My console is a day 1 Xbox1...it is terrible at upnp--it will usually get 3074 open. However, if the game requires another port, it doesn't open it's self. With that console, I have to manually port forward. There are a lot of comments talking about the poor upnp on early console on Reddit and across the web--with how long this console has been out, it makes me think that this is partially a hardware issue.
The second console that I occasionally have hooked up here is the new 4k/HDR Xbox1. It's upnp is far more reliable. I still have to reset after inactivity, and I sometimes have to try a few things to get it working on the console--
I try going off line and back on in settings,
testing the multiplayer connection,
and restarting the game.
When it's working I get rules show up under system log-port forwards as deamonware port forward such as:
3011 external to 3074 internal
3099 external to 3076 internal
3170 external to 3076 internal
There have been times when I've got in game NAT open but not the 3074 Xbox requires. If I get the in game working, I don't worry about the former. If a game uses an alternate port and I have it open, I don't have any of the issues that a moderate NAT brings, I can even connect to players with strict NAT both in game and with a party.
And finally, one note: I have Comcast service with both IPv4 & IPv6, it is possible that one console if functioning with IPv6 and bypassing the need for forwards. However, connecting to games that run entirely on centralized servers (servers rather than a player in the match is the host) seem to work far more often.in those cases, I think IPv6 may be playing a role. I don't think that's entirely what's happening here, bc most the games I play arent hosting games on their servers, their servers are only doing matchmaking.