When you use reflection spheres and static lighting in an empty environment it is normal that you dont get any reflections. Reflection spheres capture whatever is in the environment. And you cant see skylight reflections when you have a sphere background covering the mesh. The best way to test these to see the difference is make a chrome material(or get the one from starter content), put it on a sphere mesh and toss it into the level.
The sun here is movable(reflection sphere works but only as a preview, you wont see the effect in game). The first screenshot has reflection sphere disabled, and an HDR in the post process, intensity 0.1. You can see how it is filling the reflections where SSR cant. The second one is a skylight with the same HDR image, intensity 1.5. The black areas are the angles where SSR cant capture reflections, but HDR is still visible where there is no obsturction which is sky. And the third one is the same skylight with reflection sphere enabled. Reflection sphere fills all the distance SSR cant capture.
The AO solution in the PP isnt great right now so you may want to go with static lighting if you want the best AO and lighting condition. And that will require you to create a lightmap UV for the mesh of course if you havent already.