Extend antenna on Ecolink 1037B Motion Sensor?

I just got this motion sensor to place inside my mailbox, which has very tight tolerances and a horizontal mail slot. I had originally tried to put a Samsung MultiSensor in there, but couldn’t make the open-close bits fit.

The MultiSensor had no trouble transmitting through the box, but this Ecolink seems to treat my mailbox as a Faraday cage and constantly is reported as “Motion” just like when the battery is taken out, so I’m assuming it’s not getting signal to the hub. It’s not a range thing because it works fine when on top of the mailbox.

What I would like to do is give this thing an external antenna which I can run out of the mailbox, but I don’t know enough about how to read a circuit board to know where to attach it.

I’ve enclosed pictures and I think that my best bet may be P3 at the top or one of the 6 jumper spots on the right.

Any help or advice would be greatly appreciated.

Photos of PCB