The Ungwana merchant is not present at the Sunken Goldmine outpost in Skull and Bones. The Sunken Goldmine is an outpost controlled by the "Outcasts" faction, and the merchants there belong to that faction.
The Ungwana merchant, as mentioned previously, can be found at Jiwe, the main settlement of the Ungwana faction, located in the AL Royale region. Their primary focus is trading ivory, not blueprints or other items typically found at outposts like the Sunken Goldmine.