Low cellular signal after replacement housing on Iohone 6s Plus.

Hey guys.

Long story short I decide to change housing on my IPhone 6s Plus and its turn out I spend lot of money and my IPhone turn out not working completely.I damage the screen,few battery's,few camera's and eventually I got my self new phone but I want to be done with that one.

Recently I finally put everything together,new screen,camera and battery LOL and I have very low network signal.Where is actually cellular antenna on 6s plus?Maybe I should replace it?

Wifi and Bluetooth works fine but I have just 1-2 bars of network signal,compare to my IPhone that I use currently have full bar always mostly.