Converting UTM coordinates to cesium coordinate

Dear all,
I am new to cesium ion. I am currently working on an assignment, the output is to show my 3d buildings on a web map application. And I think cesium is the best platform to have it on the web. The problem is my 3d building is in UTM 23S zone coordinate, I read on the docs that cesium only supports EPSG:4326, however when I tried to convert UTM to 4326 on FME, the building didnt display properly, I have 10k buildings and the display in FME only shows 1 building with very strange height and shape. Could anyone help me on this issue?