Don't show water in UE5 by use CesiumPolygonRasterOverlay。

I throught “Cesium Cartographic Polygon” create a area,then create a “CesiumPologonRasterOverlay” node under “CesiumWorldTerrain”.In the details panel, add an element to “Pologons array” and set the CesiumCartographicPolygon.Next change the “Material Layer Key” 's value to my material’s name.
After completing these operations, the material I set does not take effect.Why?
This is my material

Before set material layer key:

after set material layer key:

The “material layer key” is the name of a particular material layer inside the selected material layer instance, not the name of an Unreal material. See step 2 (especially sub-steps 13 and 18) in the “Editing Tileset Materials” tutorial: