It works, but you have to know how.

Rwanda
Buy a MTN sim card, insert it into an other phone and call 455 to have an internet activated sim card. (For some reason this didn’t work by activating it on my iPhone itself)
The apn you need is: ‘internet.mtn’
cost: 30 Rwf per Mb (0,052 $/Mb) (see the mtn website)

Congo
Buy a Vodacom sim card, insert it into your iPhone and just set your apn to: ‘vodanet’ (I went to a vodacom shop to activate it, but I think that’s all they did…)
cost: 0,40 $/Mb (says the Vodacom site, but I think it was less expensive)