I've not tried it through shower tile, but it will go through interior walls. Easy to test since you have one in hand, you'd probably need to set it to higher sensitivity and distance, and mount it on inside of the doorway so it's facing the toilet/wall, which will again likely complicate getting power to it.
There is a battery powered mmWave sensor, might simplify your life a bit if you want to use mmWave sensors in the bathroom:
