Please follow these steps to apply a 3D LUT to an image in Affinity Photo:
1. Open your image in Affinity Photo
2. Go to Layer > New Adjustment Layer > 3D LUT Adjustment
3. In the 3D LUT window that opens click on Load LUT and choose the LUT you’d like to load.
4. You have successfully loaded a 3D LUT in Affinity Photo.