When you upload a 3D model to Cesium ion to tile it into 3D Tiles, the coordinates of its vertices are assumed to be in Earth-Centered-Earth-Fixed (https://en.wikipedia.org/wiki/ECEF) XYZ coordinates. The vertices must be in a local reference frame (centered around the 0,0,0). To place it on the right place on the surface of the Earth, you must pass a lon/lat/height when uploading with the REST API or geolocate it after the fact, see: Support tiling georeferenced 3D models into 3D Tiles.
It would be nice to add support for model vertices in other coordinate systems, that can then be reprojected when tiling. This is important when you have a model that covers a large area, where the curvature of the Earth matters. For example, a very long train track would need to curve to stay on the ground.