Soil moisture sensor not working in "pcb"
The answer was, in fact, my poor experience with these boards and how they work.

As you can see in the pictures, wire and connectors are on opposite sides of the board. I was assuming that pads were connected on both sides, which is not true. Therefore, connectors were simply dead. 

Soldered again at the wire side and it works like a charm. Thank you all for every suggestion, and the learning experience.