Hello how are you? I have the the asset tracker(AT). What I did to power it was to install the Electron in the female herders of the AT and just connect the Lipo battery to the Electron and it won’t power on.
What you need to do is to update the firmware on the Electron to the asset tracker library and then it will power on and search for GPS fix
Also I do recommend Connecting over USB and if you are a beginner it would be great if you comment on/like this post Better way to firmware update via USB 