The best is to get either a MTN or Vodacom sim card. Whichever gives you the best signal in your area.
In the shop ask for a pay as you go sim card and load anything from R50 to R150. Then ask them to enable out of bundle data. Vodacom tends to do it automatically. MTN tends to leave out of bundle data disabled unless you ask them to change it.
The tracker uses very little data per month. Out of bundle means that it automatically converts airtime into data. R150 will normally last years. Considering the hassle of reloading airtime on a sim card it is worth getting a bigger amount.
The reason we suggest a pay as you go instead of a contract or a data bundle is that it either costs more or expires. Airtime does not expire and is therefore the cheapest option.