I tried to use a wireless doorbell for landed house, but it failed miserably.

Not due to waterproof issue.

I sealed the button press unit with glue and it is waterproof as far as I can see.

But the problem I had was the battery. After using for a while, the dry cell voltage got out of operating range for the remote. I suspect it's due hot sun, the battery loses too much liquid and therefore the circuitory refused to work. When I open up the remote unit and change the battery, it works again. Multimeter the battery shows the voltage too high. It's not a case of voltage too low.