Getting throttled BEYOND Fair usage

Having a 4G LTE package I have been trying to find out WHY am I experiencing +40Mbps download and Upload during the day all day, but when it comes to the evening from about 20:00pm my download speed goes down all the to 1-4Mbps. For WEEKS I have been asking Afrihost and MTN to know why this is happening and if they are throttling me above and beyond the "Fair usage" policy which they denied. Even if the towers are busy at peak times, why do I lose 90-95% of my download speed, but remain 100% upload speed? Maybe if I was reduced by 50% download speed that could still be acceptable as fair usage, but come on, 90-95%...really guys? I can barely do anything. And NO ONE can solve this issue now for WEEKS. Why do I pay R600pm for LTE and I can't enjoy proper internet speed at night? I can't even do my Zoom or Teams meetings, because the download speed is more than useless at night. If only I had fiber in my area, but I don't...and not to mention I have full strength 4G ALL THE TIME.....this is absurd and just ridiculous.