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.

Never tried that before. Not sure if it is reliable.

The other thing to consider is the signal range. For landed, the distance between the remote and the receiver is normally much further than a condo unit.