Why does my t999 have such a weak signal?

A

AC Question

I just bought the phone cheap and just did a quick check for vital functions before I replaced the outer glass in order to resell.
it seemed like the signal was a little low but figured it was just because I didn't have a sim card in it. Proceeded to plug in my Family Mobile sim and find out it never goes over 2 bars and regularly displays the no signal icon and loses connection with network. I opened it up to ck for loose antenna coax or something else but don't see anything. This one is running version 4.3. Is there something I can do with Kies or something on my laptop?