If it’s a glTF model, the Y axis should be the “up” axis. For OBJ, since there isn’t a standard, Cesium ion will do the most commonly used choice (Z as the up axis for 3D Capture, and Y as up axis for “3D Model”).
Generally, if the size of your model is not that big (10 mb or less) then uploading as a glTF model is fine. Otherwise, you should tile it into 3D Tiles. See this thread on when you should choose 3D Model vs 3D Capture: 3D Model or 3D Capture?.
Cesium ion doesn’t currently support georeferenced glTF models (mainly because there isn’t a standard way to georefernece glTF models). What you could do is use the Cesium ion REST API to upload the model, that’ll allow you to pass in a lon/lat/height for the origin of the model so it’s automatically placed in the right location.