Alienware 15 R3 Screen replacement and brightness control

Hi,

I have a 15 R3 with the IPS 60hz G-sync. The screen is giving me headache, so i've bought the 120hz TN Panel wich is an AUO B156HTN05.1. I've assumed that since those two panel are proposed on this model, it will be ok. 

Problem is after installing the new 120hz panel, i've loosed the possiblity to control it's brightness, even if it's recognized as a 120hz with g-sync. In fact, I'm able to control brightness for a few seconds after booting the computer and it turns to full brightness after that.

Does someone have had a similar problem ?