When you import meshes it’ll appear as an array with the first element being the “root”. You’d have to use meshes[0] if you want to rotate everything.
Try this:
meshes[0].rotation = new BABYLON.Vector3(0, 1.5708, 0);
You can do this within the successCallback you provided. If you have multiple meshes you’d have to specify which mesh within the meshes array to rotate, such as “meshes[2]”. Hope that helps!