BAD IDEA
Can I put a mirror behind the router to double the Wi-Fi?
No, putting a mirror behind your router won't double your Wi-Fi; it might just make it bounce away instead.
Mirrors reflect electromagnetic waves, including Wi-Fi signals, but randomly bouncing those signals won't boost your coverage — it usually causes interference and dead spots. If you want better Wi-Fi, try actual tools like a repeater, mesh system, or simply moving your router to a more open spot instead of playing interior decorator with mirrors.