blank screen after screen replacement

i just replaced the lcd on my s5, but still i can't see anything on the screen, the notification is flashing and there are lights on the return button, so i think the phone has turned on. I am sure the screen is working properly because it works on my other s5. So what is the problem with the phone?

will that the be the lcd connector on the motherboard?

if i replace the motherboard, will i still be able to use it like normal, like receiving signal?

thanks