a little bit old but, open project settings and (search and) tick “support global clip plane for Planar Reflections”.make a material with metallic 1, roughness 0 and put that material into a plane. Put the planar reflection on the plane (if in a blueprint, put it as a child of the plane that has the right material).
warning: idk why but as my unreal engine 4 does, when I increase the size of the blueprint which has the planar reflection, it crashes. try not to do the same as I did or get a save first and then try it. goodluck