The price you mentioned for t-mobile is the prepaid price and those are always going to be more expensive
The month by month is cheaper at 5GB for the same price you are currently paying. For a short time T-mobile had a plan that had 5GB for $40 a month but that apparently was only for a short time and is no longer offered now.
Here is the link to the listing of plans:
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www.t-mobile.com/shop/pl ··· roadband