Unable to join wifi network or wrong password on ios 8.4

HI all, i m here to post my question to your attention.

i've an iPhone 5 and I'm noticing some problem with wifi. In particular my phone connects correctly to wifi modems when it is close to the modem, i mean about 3/4 metres. BUT, the iphone cant connect properly ro rhe wifi network when it s far from the mode, something like 5 or more metres. i have to say that the phone shows the networks at the maximum power signal. But when im far from the modem and i try to connect to the wifi modem, i had these error mesages: "unable to join to xxxxxxx" or "wrong password". I type the correct password, and even if i ssr my modem without an encryption, my phone doesnt connect it if far away... I have another iphone ane it connects correctly. I resetted my phone in dfu mode like a new phone, resetted the modem too, changed my wifi antenna into my iphone, nothing works.. This fault occurree some days after i upgrades to 8.4 ios. Anyone with my problem? Could it be ic wifi chip? My bluetooth works okay. Thanks in advsnce