I believe the default streaming option was in the YouTube app at one point but I don't see it any more. I believe you'll need to change that setting on the video you're watching at the time as some videos just aren't uploaded in that quality. Are you connected to T-Mobile's network or Wi-Fi when it's buffering? Could you also test other videos to see if the same thing happens?
Hey there! Just wanted to check in on the questions Mike asked above. How are things going? Have you had a chance to try out any of those suggestions?
If you have T-mobile One Plus you need to do the one time activation of the HD data pass and it will take a bit of time for the change to happen. You can contact T-mobile support for them to make the change for you Contact Us