According to Microsoft IPv6 is enabled by default but they claim it has to do with the router being used as to whether or not it'll pick up an IPv6 address. MS also likes to state that if IPv6 isn't available then the Xbox will use Teredo to tunnel IPv6 through IPv4. If Teredo is failing, that is an issue to discuss with Xbox support.
Which router are you using? Do any of your PCs have IPv6 addresses? I am using the T3200 from Telus and my PCs are getting IPv6 addresses. When I open Xbox on Windows 10 it connects with no issues at all. NAT shows as Open also.
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