Is the "Lock" switch near the bottom on the right side of the back of the camera slid to the right in the 'Lock' position?

If so, is the control you are trying to use to change the aperture selected under Custom Function C.Fn3: Others → Multi function lock?

By default, the rear 'Quick Control Dial' on the back of the camera is locked by the Lock switch. Moving the 'Quick Control Dial' when it is locked will have no effect on the setting it is set to adjust. The 'Main Dial' (directly behind the shutter button) can also be selected to be locked, as can the eight-way 'Multicontroller' (often called the "joystick").

Another possibility is that you have 'Safety Shift' enabled, along with using the camera's "slowest" allowed ISO and "fastest" (shortest) shutter duration. If 'Safety Shift' is enabled and there's too much light at the chosen (or lowest allowed) ISO and fastest allowed shutter speed, then the only thing the camera has left to "shift" is the aperture.

For instance, if you are outdoors in bright light and the camera's fastest Tv is 1/8000 at ISO 100, that may still be too much light to allow a proper exposure at f/1.8. In fact, f/2.8 sounds about right in that scenario. 1/8000, f/2.8 at ISO 100 is EV 15, which is pretty much what direct sunlight with no clouds will give you.

Even if you are in dimmer light, if you have the ISO setting manually selected at a relatively high value, such as ISO 3200 or ISO 6400, and the shutter time at a relatively slow (longer) value, such as 1/60 second, then with 'Safety Shift' enabled and 'Av' selected within the 'Safety Shift' settings menu, the camera will stop down the aperture to prevent overexposure.

Please see pages 405-408 of your EOS 5Ds / EOS 5Ds R Instruction Manual.