o customer service

I bought a S21ultra from a guy on Gumtree some time ago and as of last week the phone was blocked, after been told by MTN shops that the handset is faulty I send it to Samsung for repairs, after testing the handset they reported that the handset was 100% working and that the sim card 1 was blocked, I took the phone back to MTN Mall of Africa as they have technical staff. The staff would not assist me as I am not the original owner of the phone, they also would not supply me with the reason or letter for blocking the handset I could give to the seller, the MTN staff attitude is shocking to say the least, I guess I will move all my accounts as service does not exist in MTN