@Syl63 Try these these steps to determine your Android version.
Take a look if you can access these settings and check for your network operator.
If finding an answer is not an option I suggest considering a new budget/midrange option phone. This problem may be software or hardware as well.
@Syl63 If the card works in another phone then you have an issue with your device. Try a factory reset and if it still doesn't work then time for a new new phone.
Factory reset Galaxy S3:
I had this when I first changed to public but after a few hours it was good to go. Maybe you need to wait. That was my experience.
It would still be extremely helpful if you provided us with the make and model of your phone. What exactly is the error message you are getting? You are sure your phone is unlocked because your former provider supplied you with the code and you performed that function yourself?
Does your phone work with another active SIM card?
If your phone does not work with any SIM card, check here to see if your device has been blacklisted:
@Syl63 try changing your network to wcdma only, follow these steps:
Samsung Galaxy S3