Country lock

You must contact the carrier of that country.

See this Apple article: How to unlock your iPhone for use with a different carrier

How to unlock your iPhone for use with a different carrier - Apple Support

• Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.

Normally (in the EU) after 2 years the consumer has the right to ask that the carrier unlock the iPhone.

Return it and get your money back and buy a phone from a legitimate, authorized retailer.

Ask the iPhone's previous owner who is the carrier.

If the person you bought it from doesn't know the carrier, then most likely it was not his, and he sold you a stolen phone.

Get Your money back.

You most likely bought a stolen phone, ask the seller to give you your money back.