Bad Internet Speeds

I have a 100G (50G during the day and 50G during the night) LTE contract with Telkom. I live in the Centurion area. Since the middle of December, I have been getting very slow internet speeds on this contract. I took my Wi-Fi Router in a nearby store. The in store consultant issued me with a new sim card and checked if the software on the router was up to date. It turned out that the software was up to date. I took my router back and tried it again at home. For a couple of days it seemed to be working fine. I even went back to the store to thank the consultant who helped me. On the very same day (9 December 2018), I started having problems yet again round about (:30 in the evening. I can't stream music from iTunes without the Apple TV buffering. I can't stream videos from YouTube, Netflix and ShowMax. I pay Telkom monthly however I feel shortchanged as this is a very bad service from them. Could you please advice me on what to do because this is unfair and frustrating.