why TMobile home internet so bad

Absolute worst wifi plan ever. Have used this wifi plan for almost 3 months now and for one week it was decently good. But then It went to being worst than my old wifi plan. I have been using tmobile for a phone plan for 10+ years. I later called and they said I reached a limit of wifi usage. I later than got a new gateway and it is still very very bad. I try connecting with ethernet right to my pc from the gateway and it is still worst than my old wifi plan. Overall this is the worst wifi you can possibly get.