Hey guys, I really spent a lot of time trying to figure this out and finally after working till almost 5am on the issue I found a fix. This seems to be an issue not only with Forza but any game that requires IPv6 availability on Xbox Live, for PC or console. I couldn’t find help from Microsoft or my ISP so I took matter into my own hands.
The reason why Teredo is not available is two fold and can be due to lack of IPv6 support from the ISP (dual stack configurations that allow for IPv4 and IPv6 addressing for one subscriber). The other problem stems from the fact that after Windows 10 update (specifically 1903 I believe, but not sure) Teredo tunnel adapters were removed from the legacy device list. Not to fear if either or both are an issue as I have verified a fix.
*It is important to note that Teredo is not a permanent solution to these issues and ISPs really need to start rolling out full IPv6 support as Teredo in and of itself not only opens up many security holes but is also a cumbersome way to route traffic over any network compared to native IPv6 support (no surprises there).
I cannot take full credit for this solution as I found it on reddit here
All I did was update the tunnel domain to the current one offered by Microsoft.
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Windows Keys + R
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Type gpedit.msc
Under Computer Configuration > Administrative Templates > All Settings
Find “Set Teredo Client Port” , “Set Teredo Default Qualified” , “Set Teredo Refresh Rate”, “Set Teredo Server Name”, “Set Teredo State”.
Now, what you do is double click it, choose “Enabled” and change it to what I write below, some of these are not necessary but just change all of them :).
Set Teredo Client Port - Enable - input " 0"
Set Teredo Default Qualified - Enable - choose “Enabled State”
Set Teredo Refresh State - Enable - Choose “30”
Set Teredo Server Name - Enable - input “win1910.ipv6.microsoft.com”
Set Teredo State - Enable - choose “Client” (If after doing all steps it fails, try choose Enterprise Client instead.)
After that, Win+R , services.msc, look for “IP Helper” and restart services.
Now , Win+R, cmd, input “netsh interface teredo show state”
You should see a report stating Teredo is enabled with the domain above and your public facing IP, ports used, and your local IP and ports used. Also that DHCP is enabled with UPnP and port preservation.
You are not finished with that though.
Next you must do the following
WINDOWS KEY + R
type services.msc and run.
Find IP helper and the IPv6 to IPv4 trnaslation service.
Start the IP translation service and restart IP Helper. Wait 30 seconds at least and restart your system for good measue. When you are done you should have Teredo access and for me, that means I can finally play Forza Horizon 4 online on Windows 10. Hope this helps in advance.