Metal outbuildings are always a challenge. Rain is also a negative factor as far as signal range.
This question has come up several times in the past, and it often comes down to specific small details about set up. (I’ve moved this into the projects category for that reason, so that you can get more complete answers to your specific situation.)
I’ll post a few links to the previous discussions, reading those might give you some ideas.
You’ll see that the answers do vary depending on the specifics involved, so you may have to do some experimenting or get very specific about how things are laid out. But these should give you an idea of the options.
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. The following thread discusses specific options depending on the architecture of the shed.
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And this one is similar to the previous one, but the set up is just enough different that there are a few different suggested solutions.