You dont need a backdrop sphere if you want it for reflections. The mesh gets the reflections from the skylight cubemap(and you should set it to movable if you arent building lights.) Image based lighting will give you ambient lighting rather than specular highlights so you need a directional light. Set the directional light(which would be the sun if your HDR image is an exterior shot) depending on your image, tweak the AO and other stuff via an unbound post process volume and you should be good to go as long as your materials are setup correctly.