Our first contact was Gibson who was super helpful and informative. They have two "fleets" of fat bikes, Specialised and Silverback all well maintained and in good condition. There are several options available for cycling in the dunes. There is a shorter 12km ride, we chose the 20km. We road cycle ride fairly often in Cape Town so our MTB skills are not great.
Mark, the owner of the business led our group of 2 - myself and my wife. He was great company, never in a rush and happy to give us a few tips about riding in the dunes.
Toward the end my wife struggled a bit as one needs to "power" up the steeper sections of the dunes. As a result she fell a bit (so did I)., thankfully the sand is soft. We had asked Mark to fit cleats to the bikes. We noted he did not wear cleats. To be honest if I were to ride again, I would probably think about wearing trail shoes or similar until I had mastered the technique a bit. I think it would be easier if the sand suddenly gets soft in a steep section.
After leaving we crossed the Swakop river a little inland and headed south. The dunes are between two main roads, so there is sometimes a bit of distant traffic noise but also a lot of wide open desert experience. After a while we turned west, up and over the dunes to descend to the coast, finishing along the beach and across the Swakop river mouth.
I would not say that this is an exclusive domain for cyclists, but for those who dont cycle regularly I would recommend the shorter 12 km trip!