No service on gsm signal

Hi am having a iphone 4s which is water damaged! But mobile is now functional and now i have a serios issue! When i switch off the 3g signal mobile doesn't get signal it says no service! And when using internet through 3g network mobile gets too hot! I did all the troubleshootings but no luck. I need to know where is that gsm signal antenna located on montherboard to replace it!