How to ground wi-fi antenna outer wire on circuit board connector?
There's no proper wiring of the return path on that board. If you connected the shield of your external antenna "somewhere" to GND, you won't get an external antenna but just a long internal+external antenna wire.

Find the WiFi PHY chip (could be that one near to the existing antenna) and check which pin is HF ground. Your cable shield goes there.

EDIT: Oh yes, and Marcus is right, wire antennas are often enough not 50Ω impedance so you may need an impedance matching transformer.