TMobile Internet and Buffering

If your gateway has not updated that might also be a factor. You should check and see the firmware revision on the gateway and determine the most recent firmware revision available for the gateway model you have. You cannot initiate the download yourself. If you do find the gateway has not updated and an update might help contact T-Mobile support and see if they can push an update to the gateway. 

Use the administrative access with the mobile application to determine the primary and secondary cellular signals are both connected and ALSO the advanced cellular metrics. Posting that information to the community conversation can help as others can provide feedback. If the RSRP and RSRQ and SINR  values are poor that will be a big clue. Also confirm the cellular frequency signals for the 4G LTE and the 5G NR identification. Look to see if the 5G is n71 or n41 and the RSRP as knowing the radio signal receive power will help.

If you have not spent some time moving the gateway about to improve the signal strength and quality for the cellular signaling then you also need to possibly investigate improving the location of the gateway with respect to the cellular signal. A poor or weak signal will lead to buffering so it is important to position the gateway so it can obtain the strongest and cleanest signal. In some cases/locations the improvement of the signal can be a challenge. Not all locations offer optimal service delivery.