iPhone 6 isn't accepting other LCD-Displays

Hello together!

I've got an iPhone 6, which LCD is only showing various colors after I dropped it.

The LCD is broken. So far, so good.

I ordered a new display unit (LCD + glass), plugged it in and turned the iPhone 6 on, but the problem remains the same: it shows only various colors over the whole Display instead of showing the Apple-Logo or anything else.

So I thought I got a broken spare part (display) and tried the display of my brothers iPhone 6 on my iPhone6, but the result is still the same: only various colors instead of icons or anything else.

While both Displays (the new one and my brothers original one) are both working on the iPhone 6 of my brother, they wont work on my iPhone 6.

I already plugged the battery out and in again, but it changed nothing.

Does anyone have an idea, why the new display or the display of my brother don't work on my iPhone?

Thank you in advance!