T-Mobile, like every other cellular network, prioritizes connection and coverage over speed since being connected to the fastest network is more vital. If you are in an area where the signal strength for 5G or 4G is very poor or non-existent, T-Mobile will connect you to a slower but wider coverage network, such as 3G or EDGE. However, the implications can only be noticed in internet speeds because using the internet requires more bandwidth than calls. T-Mobile will reconnect you to your original network as soon as the signal strength reaches acceptable levels, so being on EDGE is generally just momentary. If you are locked in the EDGE network and cannot connect to their high-speed 4G or 5G networks, it indicates that there is a problem with either your phone or the network. This problem might be caused by bugs in the system or your phone, but it is simple to resolve.