If u having signal problem do following things
1 :try software mostly 80% having softwere problems in this case u can try to jail break them and them if same not working try to downgrade coz i ph3 and 3gs some time make troubles on 5.1.1 or up so max keep ur phone below 5.1.1 softwere virsion
2: check signal entina try to resold with heat gun and then clean with chemcial or patrol aslo clean golden plates with patrol better to clean all you bord
Hope u will get signals..
Regards,
Bilal68181@gmail.com