Iphone 4S cannot detect wifi anymore

What happens with my phone is that most of the time it really can't detect wifi, its stuck on the "Choose a Network" and it just keeps looking for wifi connections even though our wifi is working and my sister and my mom's phone have detected the wifi. However, there are very few times wherein it will be able to detect the wifi and it will connect to it however, after a few seconds or minutes it will disconnect and will be unable to locate the wifi again.

I have tried everything to fix this, I turned it off a couple of times, reset the wifi settings, reset the phone to new iphone, I also tried resetting it via itunes and since there are no apple customer care service establishment in our country (i think) I took it back to iStore and have them reset it there but they were not able to fix the problem.

This wifi trouble all started when I updated my phone from IOS 6.1.1 to 6.1.2. I'm not sure if this is because of the update or a hardware problem because before I updated this phone, its wifi connection is brilliant and right after I updated it, thats when this wifi problem happened.

I have not drop this phone or had it wet ever.

Is there one who can shed a light to this? Is this a software problem or a hardware problem? Should I wait for the update and hope it will fix it or is it time to throw this phone and forget about apple products from now on?

Please help me, I am very frustrated right now 😢