IYou need to create a pixel layer, fill the layer with 50% using grey scale slider and set the layer blend mode to Overlay. Select a brush and paint over the grey layer with black or white low flow low hardness. Better to use the Dodge and burn brushes on the grey layer as you can pick shadows and highlights.
There was a really good video showing how to do this on iPad but it appears to have been replaced by this one. https://affinity.serif.com/en-gb/tutorials/photo/ipad/video/286346192
You can set up screen recording on your iPad and sweep down and across from top right to open control centre to start/stop recording. Recordings are saved in Photo Album.
You have Hardness set to zero and you are painting white on a white fill layer. Delete the white fill layer. Select the ‘ Grey' layer before you start painting.
