You are correct, this is exactly the problem that’s described here: Support tiling 3D models with different coordinate systems. Cesium ion doesn’t currently support models with vertices in a different coordinate system. What you can do as a workaround is, given that Cesium ion requires a local coordinate system:
- Reproject your model to ECEF
- Compute the center of the vertices, and translate them to the center of the Earth. Call this translation T
- Upload and tile on Cesium ion
- Translate the model by the inverse of T, back to its place on the surface of the Earth.
Feel free to start a new topic here on the forum with your questions if you’re not getting the expected results and we’d be happy to take a look.