The short version
Buy a local SIM on arrival. Data is cheap, coverage is good on the tourist routes, and navigation without roaming is the single biggest quality-of-life upgrade you can make. Vodacom and MTN have the best traveller coverage; Cell C and Telkom are cheaper but patchier in the mountains and reserves.
Where to get it
At the airport — Vodacom and MTN both have kiosks at JNB and CPT. You'll need your passport to register the SIM (RICA — a legal requirement; it takes ten minutes). Or use an eSIM before you travel if your phone supports it; Airalo and similar sell SA data bundles, though a local physical SIM is usually cheaper per GB.
How much data
A 5–20 GB tourist bundle covers most two-week trips for a few hundred rand. You will not need unlimited. Offline maps — Google's offline regions, or Maps.me — save data and work where there's no signal, which is Kruger, much of the Cederberg, and plenty of the Karoo.
The coverage reality
Expect full signal in cities and on the N-routes. Expect dead spots in the mountains, in Tsitsikamma, on the Swartberg Pass, and across most of the Wild Coast. Don't rely on signal for a Kruger booking confirmation or a mountain-pass turn — download what you need first, and treat signal as a bonus, not a plan.