Hello @AOCG,
We are working on implementing a flat world terrain for use cases like this. You can track the status of that feature here: Generate MSL ellipsoid tiles as an alternative to Cesium World Terrain · Issue #817 · CesiumGS/cesium-unreal · GitHub
In the meantime, you can try the workaround described in this comment: Using a global terrain layer without height detail - #10 by agallegos
-Alex