why TMobile home internet so bad

It seems to me that they do this on purpose to save money! They hook you on the good internet for 6 months and then drag you along with these unhelpful "fixes" until you realize that it will never be the same. We all have the same problem and they all tried the same solutions and it seems to not be working. I was offered a new router and I accepted it. Been using it but nothing seems to work. I play videogames and use an ethernet cable even though its a few feet away and it still seems to have high ping and lags constantly. Debating on switching to the fiber optic but its quite expensive. ¡Es ridículo!