Note that if your mailbox is metal, it’s less of a “range” issue, and more of a “your mailbox is a Faraday cage” issue.
If you need to use it in a metal mailbox, you’ll probably need to come up with a creative solution. I saw that someone cut out a circle with a hole saw, then attached a plexiglass window inside the box and put the motion sensor directly onto the plexiglass window. Some people have also manually extended the antennas outside of the mailbox with wire.
You can see some ideas on this thread: