I’ve not played much with reflection in UE4 but i’ve done this often in 3D modeling programs. If you are using a single plane as your glass, you may need to duplicate that plane (and invert the normals) so that reflections occur on both sides of the window. In 3D, plane faces actually have a side that is outward facing and inward facing according to it’s vertex winding (clockwise or counter clockwise also seen as the plane normal). Your glass may only be reflecting on the outside because UE thinks the inside is “invisible” to such calculations.
You may also want to check (I think there is a flag) to have the material of the plan be two sided… that may work.