Sorry I can’t help you, I don’t have any experience with Samsung phones. It could be that unlock codes don’t even work with this type of phone and you would have to connect it to a PC with a cable. But I don’t know. About your first question, the lock is usually put in place by the carrier, e.g. you get a “free” phone with a 2-year contract and they want to make sure you can’t use it with a different SIM. When the contract expires they want you to trade in your phone for a newer model, and renew your contract. It’s usually possible to have them unlock your phone at the end of the contract, sometimes for an additional fee. So it looks like you ended up with a phone that is still locked to a certain carrier and you can only use SIM cards of that carrier.