Weak Wifi, replaced phone from apple

hello,

my iphone 4s was replaced at an apple store long time ago.

i never used it. now im thinking of selling it and i recognized that

the wifi is only working when im next to the router.

it had ios 7 , i updated to ios8.4, but still no change.

i had a look and retightened the screws but still no use.

after research im not really sure if its a hardware or a software problem.

after all its replaced and the hardware "should" be ok.

any help?