iPhone won’t work with my SIM card?

i bought an iPhone 6s on offerup. It is not unlocked, it's for T-Mobile. I’m trying to use a T-Mobile SIM card with it. My SIM card works and the iPhone SIM card reader works. The phone is not locked by the carrier I have called them to see if the previous owner was still in a contract or didn’t pay it off. There is no problem with my SIM card or my T-Mobile account. I can set up the phone and use it without a SIM card, but as soon as I put my SIM card in it says activation required and goes to a setup screen, then it says sim not supported. The phone isn’t iCloud locked or anything.