Extremely bad service

I’ve been using MTN for the past 20years but they have never solved any enquiries whenever I call their customer service line and I let it slide coz it was small amounts of airtime now my 50gb disappearance and they say because I changed price plan I can’t get it back. My problem is why don’t they inform customers that everything will disappear once you change, I’m so frustrated and the customer service was also bad as they kept on hanging up on me . I even want to port to a different network