Take your S9 to a T-Mobile store. The SIM in the S5 won't fit. The S9 takes a smaller SIM. They should be able to set up the phone also.
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Check to see if your current Sim is a 3 in 1 Sim. If it is you just need to pop it out of the holder to fit the new phone
If not you will need to follow dc5fan's suggestion.
thx - Brian