SIM unsupported on iPhone 6s

I recently bought an iPhone 6s from a friend and set it up as usual with no hiccups. However, when I inserted my T-Mobile SIM card from my current phone, I was greeted with the hello setup screen prompting me that my iPhone had successfully updated to iOS 15.7.2. After clicking "continue", it showed me the activation screen saying "it may take a few minutes to activate your iPhone". A few seconds later it showed me a screen saying "SIM not supported" with no way to exit that page (it goes away when you remove the SIM card). I looked on the T-Mobile website and inserted my IMEI code to see if my iPhone was locked under T-Mobile. It told me that my iPhone works with T-Mobile SIM cards. But when I insert my own T-Mobile SIM card, it does not accept it.