A metal switch box with a plastic switch plate typically still allows the signal out through the plastic, so usually you’re OK. You may get a signal drop of 15 to 25%.
A metal switch box with a metal switch plate, however, may cut the signal by 75% or more, and typically causes a lot of problems.
It also depends on where the next nearest repeater is. If there’s an outlet in the wall nearby that is also a repeater, that may be all you need to get the signal back to the network.