Mount a board on the wall and then mount the screen on the board.
Get an 8 foot cedar or pressure treated 2x6, you can then use timber screws to mount it to the wall insuring you hit the studs. You will need to pre-drill both the 2x6 and the cement siding, the hole in the siding should be as big as the screw and the hole in the board should be just smaller then the screw. This will prevent the board and siding from splitting.
I would think three studs would be sufficient for the weight of the screen.
Put some Quad silicone in the hole in the siding and the hole in the 2x6 to prevent water from entering the wall.
You may want to use spacers behind the board, cedar or composite shims also pre-drilled, so that the board sits off the wall by a 1x4 inch. This will allow air to get in and prevent water getting trapped, it will also help to put the board at the correct angle if your siding is typical lap siding.
You now have solid surface to mount your screen to.