Hi faryaz the next step i would recommend is a reset all settings
settings > general > reset . reset all settings
This will not delete any of your data but it will put all of your settings back to factory settings ( i.e. ringtones, backgrounds, etc...)
test this out and let me know how it works
Its best ot do all the software troubleshooting before jumping to hardware replacments
Hello COD Lover thanks for replying I have done this but problem is still there, I have restore iphone 4s in DFU mode as well but problem remain same.
Also when I have done DFU mode restore after restore when iphone start my lcd was flickering for 5 min and then It was OK but signal problem is still there.
If you have tried a restore as new expecially in DFU mode you have done all the software troubleshooting neccesary this narrows it down into a hardware issue.
Apple does not recommend doing any repairs on your own because it will void your warranty however
You would be looking into repairing your Signal Antenna
Before doing this repair i would suggest contacting your carrier and having them reset your cellular network just in case
Let me know how it goes
There is a possibility the logic board could have a short in that but that would be hard to determine without doing the actual repair. Even though the price on repair for the antenna was fairly cheap you will find the price for a logic board repair is quite a bit more. I would bring it back into your repair shop and see what they recommend at this point.