Metal free zone for WiFi antenna
If you want the antenna to transmit through the sheet, the metal will need to be removed. For almost all metals, the skin dept at 2.4GHz is less than 0.1mm, and even lower for 5GHz. The skin depth is the amount of metal it would take to attenuate the signal roughly 1/3rd. If you want to transmit through the metal, it will need a metal free zone. 

It might be easier to put an access point (or antenna from the same AP if it has that option, you can get extension cables for most antennas with minimal attenuation) on the other side.