IPv6/IPv4 dual stack Xbox One connectivity issues

It might be that your ISP is using IPv6 and tunneling IPv4 via CG-NAT and DS-Lite.

While IPv6 is generally a Good Thing to have (as the Xbox One primarily uses IPv6, and even if you only have IPv4, it will use IPv6 tunneled through IPv4), you might want to check with your ISP if they offer you a "Dual Stack" connection with IPv6 and a
real public IPv4 in parallel.

There really isn't much you can do on the console itself, as it does check automatically what IP protocol your ISP provides, and if it's IPv6, then the console will use IPv6. If it's a Dual Stack, where you have IPv4 natively (not tunneled through IPv6)
and IPv6, then the console will use both, and if it's an IPv4 connection, it'll tunnel IPv6 through IPv4 through Teredo.

"my Comcast SMCD3GNV wireless gateway/router/modem/telephony modem ".....those are not good for gaming imo , not xbox 1 friendly anyway...anytime you use your isp's equipment instead of your own its a gamble , they have their own firmware and (somewhere
, in fine print , you agreed to this) they can change things around , usually , most people don't notice , but gaming consoles sure do ....you can fiddle with it , and maybe get it right but I would be surprised if it stays that way ....get your own modem
(NO GATEWAYS) and own gaming router that does UPNP (netgear is good) and forgot about all the DMZ and ipv6 and all the other stuff....been there ...

Go to Best Buy and pick up a Motorola Surfboard modem and a good router. I have a LinkSys "max speed" (model EA7500). Go home, hook up the cable modem and plug a computer directly into it. Go to internet browser and you will hit a comcast page for activation.
It takes about 2 minutes and your modem should be all set, you can return theirs the next day and save money every month.

Once the modem is done just unplug the computerand plug in your new router. The setup on that should only take about 5 minutes and is very basic. Once it is done just go turn off IPv6 and then have fun gaming.

In regards to the "we have to use OUR modem with a phone connection for 911", that is complete BS. I havent used one in years and argued with Comcast about it till they gave in.

If they (or you) can bridge it , I would defiantly try that...

@RhinoTheGreat87 hey hey

I know that this not an answer to Your questions, but the best chose for You is to use only UPnP, so Xbox One would open ports for it without manual configuration. Right now You do his job, DMZ is bad chose for security, because Your Xbox One is connected
thru DMZ directly to internet, in most gateways/routers without any firewall's at all.

As for Your questions, this not really easy area, if You like I can provide to You some links about IPv6 and hot it works, but my suggestion, just use UPnP, it's great answer to all Your questions.