I thought it may have something to do with not understanding something about the process, but a little search around the internet seems to confirm that this has been a nagging problem with Mantaflow for a while.
When rendering with either Cycles or eeVee, all that shows up in the render is the domain box with no smoke. Of course, setting the domain to be unseen by camera shows nothing also. Is there something I’m missing about how to render Mantaflow smoke?
If you set up the material manually, be sure that you are using the density attribute and have the material plugged into the volume socket.
Also, you could try (in a new scene) setting up a test simulation by using quick effects in the object menu to see if that will render in Cycles at least. Quick effects will automatically set up the domain material. Since that’s what I did to test it just now, it should work.