Bad Service Provision

I took a Contract with MTN on January 2020, For 200 I get 40 gigabytes of data divided into two for day and night, My data didn’t last me a Month. On the 10th of June I got a call from MTN offering me a free migration to higher plan were i’d get 80 Gigs for 300 I accepted. The lady who was meant to assist me didn’t send me a Copy of my new contract. End of the Month my bill had increased to 300 yet I’m still getting the same old amount of data. I called Customer care multiple times they say the don’t deal with such issues, every department they transfer me to claims not deal my kind of query.