With an iron box, it also needs to be grounded, its shielding principle is to first receive electromagnetic waves by the iron box, and then into the ground, but the effect is not particularly good, we can find by train, the mobile phone signal is still full, only in the tunnel when the signal is completely interrupted.
So, when you are not using the shield, find a room with no Windows around or small Windows, or iron doors, and walls of reinforced concrete structure, so that you can screen.
A magnet or something could block the chip's signal