There is some reasonable 4G reception, but only with the South African provider MTN (not Vodacom or Cell-C) and there are a number of options you can consider.
If you have an international roaming option with your home provider, that covers South Africa, this can be a good option. But make sure that your home provider will allow you to select MTN while you are in South Africa and also check how much data you are allowed to use. This can sometimes not be the most cost effective option, but can be the most convenient, depending on how flexible your home provider is.
An alternative is buying a physical SIM card on your arrival at O.R. Tambo Airport in Johannesburg, there is an MTN shop as you come out of the baggage collection area on the right, just around the corner from the statue of O.R. Tambo in the centre of the arrivals area.
You will only be able to use a physical SIM card if you have a SIM FREE phone, ie one that is not exclusive to a mobile (cell) provider in your home country.
Alternatively, you can use an app, such as Ubigi, which enables you to install an e-SIM on your phone and buy a voucher that will give you access to data in South Africa. But because make sure that you are able to select MTN as your provider in SA.
Here is some information on this topic: https://support.apple.com/en-gb/118227