Massive fluctuation of internet speed

I would have thought if something was blocking the signal I wouldn't have 175mbps 3/4"s of the time. If something is blocking the signal, then somethings in the way all the time. I believe T-Mobile has oversold their ability to supply a steady internet signal. The more time that goes by the worse my speed fluctuates. The one thing that made the biggest difference is when I put the device on a ledge outside of a window that has a 2 ft over hang and is 20 feet up in the air. That at least made it where I could get a descent signal. T-Mobile says they do not have a exterior device, maybe they need one.