my iPhone doesn't connect to wifi

I've purchased an iPhone 6plus specifically on 21/7/2015 and it was working fine and still is until from 1 or 2 months or so the internet became really slow and almost not working most apps don't load except for whatsapp and even whatsapp is really slow and takes ages to start connecting i still haven't installed the new software update which is the iOS 9.3.1 i don't know if that's the problem..i'm also sure that the internet is okay as it works perfectly fine on my laptop and on my brothers' phones which are also apple i-phones but mine show that its connected to wifi but doesn't work and doesn't load any page or app except rarely the internet works on it for not more than 10-50 minutes..other than that its in perfect shape the screen never broke and its fast and working fine but for me its useless now as the internet is the most important feature in any mobile..i can't buy another one as i spent a lot of money on that one and i also cant find a a place to fix it as we have no apple stores or centers here in my country..i was hoping for you to kindly give me any suggestions on what i should do to fix it as i dot know what caused this problem and i'm too upset about what happened and never thought an i phone will have such problem so quickly before even a year of usage..i can't use it as much now and currently using my laptop which of course is so much harder than using my phone 😟 i hope anyone from the apple company could care and actually answer me and suggest anything because i'm helpless please and i need internet on my phone ! 😟