[HELP] Network problem after installing new LCD Display topic

I recently bought a new LCD (LCD+Digitizer+Frame+buttons) from aliexpress for my Xperia Z. I disassembled the phone & installed the new display myself. It works perfectly. There's only one problem - it cannot connect to any network at all. The only time I get a signal (1-2 bars) is when I get out in the open. This is the same while in my car also. Other than this, everything else works perfectly (wifi, cameras, mic, earpiece, speaker, etc.)

I tried using the same sim in another phone (Galaxy Star S5282). It gets full signal, wherever I go. I also tried using my brother's sim in the Xperia but same results. His sim also works perfectly in all other phones.

I had disassembled it yesterday again & couldn't find where the fault was. The sim tray is fixed to the mainboard & there are no wires/flex cables to be connected.

Any help on this will be greatly appreciated.