WiFi Module inside a metal body
Generally, if your metal enclosure is grounded and thick enough, congratulations, you might have built a faraday cage and no RF enters or leaves.

In reality, the thing is probably not a perfect Faraday cage, and you're just greatly reducing the performance of your Wifi by abusing the enclosure as a very bad antenna element.

So, yes, a plastic window would sound wise.

Also note that unless you really seal the enclosure in a dry state, you might want to ensure that condensation doesn't happen on the electronics. That can be a hard problem for outdoor electronics, depending on the climate.