Giving old iPhone to sister. It’s unlocked but won’t support her sim card

its saying sim not supported. the message currently says the same card that you have installed n this i phone is from a carrier that is not supported.

this is my older device, iphone 6Splus, which has been unlocked for a few years now and I have gifted it to my sister in a different country. She has restored and logged into her iCloud account but her sim is not recognized and it’s asking for us to contact carrier to unlock it..the phone is unlocked tho..what can we do?