The IMEI should be the same. I'm not sure what you mean by information button, but go to Settings -> General -> About -> IMEI. That IMEI is the one that you should check and it should match the back. Also SIM not supported doesn't mean activation lock, it means NETWORK lock. If you signed into the iPhone with your own iCloud already, it may show activation lock on. You need to check the IMEI to see which network it will work on, because it is not unlocked from the network.