Phone manufacturer prevents installation of apps signed by third paries, does anyone know a way around this?
Not sure how it works with LG exactly because I don't own an LG phone. But I had similar issue with my mi phone.

Before you can install any apps using adb you need to enable developer setting allow usb debugging, and allow install via usb. LG might have some other flags too

In older version of android there was a switch - "allow install from unknown sources". Please look for these settings and do the needful.