How to know which iPhone model I have?

Then you have a much bigger problem than just not knowing the model iPhone you own.

If the previous owner didn't prepare the phone properly for a new user, you're now the proud owner of a useless brick, you cannot use.

Hand the phone back to the previous owner and have them follow the directions in this link --> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

If they are unable or unwilling to do this, get your money back. If they won't give you your money back, recycle the iPhone responsibly. Apple will NOT help you bypass a locked device you were not the original owner of.

Read this link as well --> The All Too Common SAD Reality of Buying … - Apple Community

Apple is simply not going to help someone who is not the original owner bypass the passcode, or Activation Lock without the original purchase receipt. The OP doesn't know the Passcode. I'm willing to bet they don't know the Apple ID or Apple ID Password on the device either.

Restoring it will remove the passcode and doesn't require Apple's involvement. The question is whether or not the unit has an activation lock; if it doesn't, they won't be asked for an Apple ID or password.

(224829)

That's an iPhone 11. To check other serial numbers, click here.

(224813)

Open the Settings App on your iPhone. Tap General. Then tap About. The model you own will be listed there.

Contact Apple. If the unit isn't locked, you can just restore it.

(224825)

They don't know the passcode, so how could it not be locked? Apple isn't going to help them unlock this iPhone.

The passcode lock isn't the same as the activation lock. Apple can check the activation lock status.

(224826)

OK.....l'm not going to argue with you. I'm pretty sure they've managed to purchase a useless iPhone.