Hey guys.
(*Below, in a nutshell... I'm long-winded*) I COULD NOT CONNECT WITH IPv6 CONNECTION! Call ISP and ask if they can switch it to IPv4, or if there's a setting on your router to switch between the two.
The above solution tells how to get this done (Play,Head to Head then create or join session), but for those of you who are trying to connect, and have no luck, your internet may be the problem.
I know what you're saying, "I have great internet! You're full of it!" but my problem was exactly that. My internet was too good (IPv6). IPv6 is NOT COMMON in European countries and made it so I would join games and people with IPv4 would instantly disconnect or my game would freeze.
I spent about 6 hours trying to connect, only to have no success, less hair and no achievement. My friends with inferior connections were joining games left and right and I continued glaring at my TV screen. After hours of messing with my settings, making sure my Xbox NAT was open, still no success.
I called my ISP and they said they couldn't change it on their end and I was unable to change it on my end, as well. I had to get a new router to be able to have a toggle switch to turn on one or the other, or both. So after spending 3 days messing around, I hope I can at least save you a step or two.
If you're having connection issues, go to Settings>Network (Settings). The type of connection I needed was IPv4, but I was using IPv4 & IPv6
I really hope this helps someone, or I shall suffer the thumbs down of Doom...