In general, any component with RasterOverlay in the name is meant to be added to a Cesium3DTileset actor. Otherwise, it won’t really do anything for you.
I see that you have both Cesium World Terrain and Google Photorealistic 3D Tiles in your level, so you may want to remove one of those tilesets. Then add the CesiumPolygonRasterOverlay component on the remaining one, so that it can work with clipping.