UE4 combines some techniques to generate reflections and none of them are perfect. By default you get Screen Space Reflections (SSR) which uses everything you’re seen on your screen to generate some reflections. The results are relatively precise but you wont get any reflection of things that are not appearing on your screen (say the buildings behind you). When you add Reflection Capturers (comes in sphere and cube flavors) you’ll get a baked 360 texture of the environment combined with SSR. The transition between SSR and reflection capture will never be perfect and won’t work at all for mirror-like surfaces. Then you have the Planar Reflections which may be the best option for you.
Take a look at this: Reflections | Unreal Engine Documentation
and this Planar Reflections | Unreal Engine Documentation