See http://www.freerepublic.com/focus/f-news/1381928/posts?page=32#32
for larger scale map with roads. You'll see that there are roads that connect Uige via DRC. Since both Uige and Cabinda have coffee plantations i assume that there is a fair amount of contact between the two. Although I would also be worried about a coastal freighter between Luanda's port and Cabinda. I don't know where in Cabinda the cases are.