This is not a product advertise, but you might be doing something wrong: https://www.youtube.com/watch?v=ayROuQyXI6k
Couple of things to check:
- material for the mirror: 0 for roughness, 1 for metallic, color = white
- planar reflection: check details and set screen percentage parameter to 100 (default is 50)
- add reflection capture to the scene spot