Here's what I found on a review site of your laptop:
"The Latitude sports two graphics cores: Intel's HD Graphics 4400 GPU and AMD's Radeon HD 8850M graphics chip. The Intel core is responsible for routine use and battery mode. The Radeon GPU is enabled when high graphics power is required, for example in computer games. Which GPU is actually used is determined via application profiles. The user can always change the settings in the driver menu. The Radeon graphics core belongs to the upper GPU midrange. It supports DirectX 11.1 and works with a speed of 575 MHz, which can be increased up to 725 MHz via Turbo."
It sounds almost like you will have to trigger the discrete graphics card to be used by setting up "application profiles", but I can find nothing written via a quick Google search that explains in any way how to do that. I always thought that the discrete chip would be triggered automatically when needed -- at least that's how it works on my Dell 15z with discrete nVidia graphics.
Do you a user guide from Dell that mentions switching graphics?