Getting 4G/LTE signal through sheet metal - opening/aperture size needed?
This answer, regarding metal shields, included math from Richard Feynmann's lectures.
He spoke of wire mesh with specific spacings between wires.
For the field inside the mesh (aperature in a metal box), the field drops off
exponentially:
Why are many IR receivers in metal cages?
The useful point about this (years old) answer is this

you need to place the Receiver (or Transmitter) close to the hole in the shield, or the severe attenuation ruins the communication link

Summary: if the hole is X size, and the router is X distance INSIDE the metal box, the attenuation is
e ^ (X/X) * 2 * PI = e^6.28 = 1/533 (21 dB if power, 42 dB if voltage)
Thus you either need a BIG hole, or locate the router to be very close to the hole.
Because of &quot;reciprocity&quot; of transmitter fields and receiver fields, if you have 15 dB attenuation of TX fields, you will also have 15 dB attenuation of RX fields.