Their website is below. No problems with my SIM card here in the US. It roams on AT&T. You can also try their Facebook page. What's the problem you are having and where are you located?
http://www.mtnonline.com
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile