Garamantia in the area of the greatest road penetration was a major trade partner of Rome (and I think Septimius Severus may have vassalized them?)

I don’t believe that the climate was the difference. One of the big differences, though, is that increasingly complex and ingenuous systems of irrigation (by African societies, not introduced by Rome) both heightened the living standards of the region and steadily drained the fossil water trapped underground in the region - supplies which, unbeknownst to any society at the time, were non-renewable.

So their success reversed itself over the course of hundreds of years, unfortunately.

Oh! Also, you may be interested in the course of Roman military exploratory expeditions in Africa