Iphone 6 with wifi-issue (not slow, but lost signal constantly)

I do not sure Is this the iOS problem or the device problem. Me and my cousin we both have the 6 and have been in same wi-fi network. His phone is doing well never lost or drop signal. Mine keeps dropping wi-fi connection, and when I try to go into setting to reconnect the wifi. It shows nothing on the wi-fi list. I have to reset the iPhone to be able to get back on wi-fi again then after a while wi-fi will just disconnected and not be able to search for any wi-fi network again. I have done full-restored thru iTunes. It helps a bit for a good day, and the issue comes back again.

Is anyone encounter this issue? I only have seen people having problem with slow wi-fi network, but I didnt see anyone addressed this issue yet.

 

Have done all of that. It still doesnt solve the issue. It's not like the router problem because there are plenty of wi-fi networks on the lists, but when the phone drops the signal. It seems like the phone can't catch any signals at all.

Should i take it back to Apple and let them replace it? I think if it's iOS problem, replacing phone is not gonna resolve the issue.