A long time ago I read about a Roman-period source that said that a group had gone south into the Sahara and had passed through places including Cadamus (= modern Ghadames) and had reached a river called Dasibari. Please who wrote it, etc?
Plin. Nat. 5.11 is probably what you are looking for.
If this account is true, then Romans reached the Niger river: compare Dasibari with the Niger’s Songhay language native name Isa Ber. (I read somewhere that another version of that native name is Da Isa Bari = “great river of the spirits called the Da”.)