3D model apear flat not showing shades

Hi, thanks in advance for the your time.

I uploaded a “.glb” model to cesium.
This is how the model should have been rendered:


And this is how cesium is rendering it
image
basically, it does’nt render shades and the model looks flat

I’m using SahrpGltf to generate the model from some topographic data.
In my code i’m using MetallicRoughnessShader.

hope you can help me, thanks in advance