That's my speed on a very good good day, I used to average more than that though back in June/ August 2018 I'm talking at least 30 to 40mbps on a very good good day
My exact same experience. Awesome in the beginning, terrible the last couple of months. And that's why I cancelled mine this morning...
What really frustrates me is that apparently anything above 4mbps is considered acceptable. (WebAfrica informed me they will only log a fault if my speed is less than 4mbps.) RAIN should be fully aware that the word "LTE" has some expectation attached to it - There is the expectation of getting speeds well in excess of ADSL.
Good luck with getting it sorted. WebAfrica told me that it's most likely tower congestion. (It wasn't RAIN's official response, but the fact that WebAfrica suggested/guessed it is the problem, probably suggests that they are used to these types of complaints and that congestion is an issue at a number of towers/areas.)