The short answer
Expect very little. At Erg Chebbi you may catch a flicker of mobile signal at the edge of camp, and most camps deliberately have no Wi-Fi. At Erg Chigaga there is no signal at all once you leave M'Hamid. Agafay, close to Marrakech, usually has better coverage. Charge everything before you ride out.
In more detail
Our Merzouga notes say it straight: a flicker of signal at the camp's edge and, on purpose, no Wi-Fi at most camps. For Erg Chigaga, signal stops in M'Hamid town and the camps are intentionally off-grid. It is part of what people go for, but it helps to plan for it.
Tell family or colleagues you will be out of reach for the night. Download maps, music and anything you need before you leave the last town. If you rely on a phone for medical reasons, mention it when you book so the camp and your driver know.
Power is usually there for charging at luxury camps, by solar or generator, but sockets can be few and shared. A charged power bank is the simplest fix. For staying connected on the road in between, a local SIM or eSIM covers the towns and much of the route.