I am trying to replicate the example shown here, made with rayshader package:

If you want to save them as .png, you're using the right function, but you've to open the rgl window, i.e. first launch the plot, then save it. Something like this:

plot_gg( mtplot
        , width = 3.5
        , height = 3.5
        , sunangle = 225
         )

render_snapshot("path\\to\\your\\folder\\plot1.png",clear = TRUE)
# close rgl window
rgl.close()