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I got a masterplug weatherproof electrics box to house a mesh ap. It sits a few metres from the back patio door within range of the mesh ap which is just inside the back patio door to relay the signal down the garden. I get a good signal at the far end of the garden c.30m away in the garden office. The setup is needed to get through a patio door with some kind of special glass that knocks the stuffing out of the wifi signal.
You could try similar with a mesh point either side of the metal wall.
How does the power cable get inside?
You could use a weatherproof AP mounted on the outside if you really can't get anything into the structure.