As you are discovering the electronic versions are all designed to be in protected environments where the battery and brains of the lock are protected from the elements by being inside. I have only used the mechanical locks successfully when both sides are exposed to weather.

@HALLETT & Co. You would think with smart locks being so incredibly popular for the front door, that some company would realize this is a market opportunity for them. If you did not want to harden the smart lock itself, you could probably design a small controller panel that goes up against the rain-protected side of the fence, and run a small conduit from that to the smart lock. But so far that product does not exist.

I started a new thread here on this topic if you want to participate there:

https://www.houzz.com/discussions/6212967/anyone-make-a-smart-lock-with-push-button-codes-for-side-yard-gate