Device/SIM card

So I purchased my iPhone 11 Pro Max through at&t. I recently requested for it to be unlocked so I can use sprint services and they approved it on July 13, 2020. I received the instructions for using the sprint sim card But when I put the sim card in it’s says “sim not supported”. I heard multiple things saying I had to wait at least 24hrs before using the SIM card but I’m clueless at the moment cause it’s still not working. What’s are y’all thoughts? Can anyone help me?