Hey guys, I have a problem with my unifi wifi connection. I hope someone can solve my problem ASAP!

First of all, I'm not so sure if I'm posting in the right section, my apologies if I should have posted this somewhere else.. couldn't find anywhere better to post
So a few months ago, I bought the ASUS RT-N14UHP router since my TM Unifi router had a problem, and to start off it was a really really awesome router.
But about a month or so ago, I started facing problems with the connection on my iPhone 5S, it was roughly around the period of time where I just updated my phone to iOS 8.

Now, the internet connection on my PC, with the LAN cable has no problem at all, but on my iPhone, I realized that sending messages in Whatsapp took really really long to get
a tick, and sometimes not even send at all. Also, loading on Instagram takes quite long, also sometimes will not even load at all. It definitely was not like this when I just got the router.
Also, speedtest on my phone shows low ping and also normal download speed, the only thing I noticed is that it takes super long to send whatsapp messages, and to load Instagram and Twitter feed.

Note that this doesn't happen when I'm using cellular data, and also my brother's iPhone 5 (still not upgraded to iOS 8) and iPad faces this problem too.

I've been constantly restarting the modem, router, changing the wifi password, restarting my phone and I also called unifi to check and they said that they refreshed my port and connection.
I've also restored my iPhone to see if that would work. Nothing changed
Sometimes after restarting the modem, router and my phone, I can get it back to normal for awhile, but the problem would still persist.

These are my wireless settings for the router



I'm guessing that there's something to do with the settings...

I'm hoping someone can help me out!!!

Thank you!

I'm sorry but what do you mean by 2.4ghz and 5ghz? Sorry total newbie here