Which of the two Unlimited plans was selected? ....gounlimited or beyondunlimited?
gounlimited subscribers are subject to throttling and/or deprioritization at any time, especially if a tower is experiencing congestion. Video quality for smartphones is restricted to 480 while tablets get 720.
beyondunlimited acts like the former Unlimited plan with throttling and deprioritization kicking in after certain softcaps have been exceeded. Video quality maxes out at 720 for smartphones and 1080 for tablets.
All of this is dependent upon having a good signal/connection to a cellular tower.