Falsely advertising a product they don't deliver

Mtn South Africa are selling a home LTE plan and they promise speeds of up to 15MB. I've never had speeds exceed 2mb at any given moment and it is not a coverage issue. What's worse, their FAIR USAGE POLICY clearly states once u use up to 180 gig, speeds drop to 512 kb/ps. My speeds don't go past 136kb/ps. When I contact their social media teams they instead tell me to upgrade my speed. They are selling a product they are not delivering. Why are speeds far less than those on their fair usage policy? Calling their call center is a waste as nobody is even willing to assist. Absolutely appalling service. They are essentially selling u a 200 gig package because they know once u clock 180gig, the rest of the month will only leave u accessing a maximum of 20 gigs. This is very disappointing