SIM not supported on unlocked phone

Hi,

I bought an used iPhone X but when I put my SIM card pops up the message "The SIM card that you currently have installed in this iPhone is from a carrier that is not supported ... “ which means that the phone is locked to a specific carrier. I called the carrier of the old owner and I’ve been told that the phone is actually unlocked. Then I payed one of those companies that check all the info about your phone based on the IMIE-number and they confirmed that the phone is unlocked ... I have, of course, tried all the simple things like restating the phone, rebooting it etc and I have checked with my current carrier that SIM card is perfectly working. What is the problem then? Any other ideas?