Bought unlocked phone but sim not valid
I was in US on vaccation and i have bougth three Iphone X pro for my family. I bought it from one of the Best buys in Boston. I have intentionally asked the sales agent the i want UNLOCKED phones. When i get back to my country and try to use it, it keeps saying "Sim not valid" with long message. All of them have the same issue.
wha was i supposed to do?
Thank you for your support
iPhone XS