One of the many new features in iOS is something call WiFi Assist. What this basically does is when you are surfing the internet and is using Wi-Fi, iOS 9 will “automatically use cellular data when Wi-Fi connectivity is poor”.
For a more detailed explanation – watch this WWDC video on Your App and Next Generation Network and jump to the 17min mark.
In Singapore where many of us stay in HDB flats where your broadband or fibre point is at one end of the house, and say your second favourite place to surf is in your bedroom-toilet, unless you have multiple Wi-Fi routers/repeaters to provide maximum coverage, chances are you might encounter less-than-ideal surfing connection there.
Or maybe your home router is wonky – even if it is showing 5 bars on your iOS device, you find that you are not connecting to any site.
With iOS 9 and with this feature switched on by default, when the OS detects that your Wi-Fi connection is not connecting, it will automatically switches to your cellular network to provide you with a seamless experience – which in most case is a good thing. But if you are on limited data plan like most of us, you might end up bursting your data limit before your next cycle without you realising it.
So to be safe, go to your Settings app, click on Cellular and scroll to the bottom of the page where you’ll find the toggle for Wi-Fi Assist. Switch it off. It will probably make you a happier person when your next mobile bill arrives.
Alternatively – if you want to keep having this feature on and have better connection regardless of Wi-Fi or cellular, what you might want to do is to decide which app you allow to connect via cellular connection, just so you won’t happily watch a Netflix movie if your Wi-Fi router decides to fail you.