Hi @Haadi_M,
You can position your glb content relative to the ellipsoid, terrain or 3DTiles layer.
Based on the previous conversation in this thread Cesium Clip Terrain is Blank - #2 by Luke_McKinstry, you may be setting the height for the glb with heightReference: Cesium.HeightReference.CLAMP_TO_TERRAIN. This positions the glb on top of the terrain. Most users prefer this setting as it aligns the model with the terrain.
This sandcastle example shows alternative ways to position a model Cesium Sandcastle One option if you want to position it below the terrain, you could supply a height value that is less than the terrain height at the location.
Please let us know if you have further questions.
Best,
Luke