double nat on console home internet

Run a scan for ports that are supposed to be open.

BY DESIGN their XLAT464 approach BREAKS this functionality.

https://datatracker.ietf.org/doc/html/rfc6877

From the introduction:

 This document describes an IPv4-over-IPv6 solution as one of the techniques for IPv4 service extension and encouragement of IPv6 deployment. 464XLAT is not a one-for-one replacement of full IPv4 functionality. The 464XLAT architecture only supports IPv4 in the client-server model, where the server has a global IPv4 address. This means it is not fit for IPv4 peer-to-peer communication or inbound IPv4 connections. 464XLAT builds on IPv6 transport and includes full any-to-any IPv6 communication.

So long as an application in question is reliant on unsolicited inbound v4 connections, their dual stack solution BREAKS THEM BY DESIGN, because they have yet to take proper measures to guard against it.

For example, I just set things up locally... even dropped some security to allow pings and whatnot, and this is the result I got testing to the first one of the UPnP requested ports for my PS4:

Port 9308 does not appear to be open.

Even connecting the PS4 directly via ethernet fails.

When running through my router, nothing registers as an attempt being made.  Not even a basic ping attempt registers.

TMO's topology clamps down on things before the packets reach this layer. 

My PS4 THINKS  I am on  NAT-2, because UPnP is negotiating the rule to open the port... but when something tries to actually get through directly on that port, it fails.  Because their topology does not support such inbound communication.

Only ways known so far to get around this is via a separate tunnel that allows such p2p traffic (like a GVPN and such), actual functional v6 delegation instead of relying on v4, or for the application in question to instead use a stateful v4 connection type like the typical more dedicated TCP server connections.

But the more direct UDP and other P2P models a lot of games have started integrating (and that consoles actually use in the background), it breaks certain functionality.