[tsac]

The video quality is mostly based on the bandwidth the carrier is supplying to users.
Sadly if the carrier is experiencing heavy use in your area you will see poor quality.
ATT is doing some limiting depending on your usage. I have noticed the display change from 4g LTE to E on my phones indicating speeds as low as 217kbs. Your phone can go to GPRS which is 56kbs ( very low speed)this indicates the carrier is limiting speed and potentially the quality your receiving.
You might also see if you can use the mobile version of YouTube to see if you get any improvement. Generally users say the have good video display when using the 9900

The other thing you can try is to check with ATT to see if they have issues in your area but don't expect them to tell you they do. It might help to ask other ATT users and see how they are doing.