I don’t think the white wire should be plugged into SN2 — SN2 is for a second sensor, it’s not for the common wire of the first sensor. If you checked the user manual, a sensor should be connected between SN1 and GND (or SN2 and GND if you have a second sensor). I don’t know how this particular sensor works — I would assume if you measure the resistance between the brown wire (normally closed) and white wire, it should be 0 when no rain is detected, and should be open circuit (infinity) when rain is detected. If this is the case, then the white wire should be plugged into GND.