Reboot Router Or Modem
It may sound like a fairly simple solution, but sometimes the problem isn’t actually with the device or the installation of iOS itself.
Your first port of call should be to ensure that the modem or router in the property is rebooted fully to see if that resolves the problem in the first instance.
Like all tech problems, there is not harm in starting off by force restarting your iPhone or iPad to see it helps.
Hold down the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the screen goes blank and Apple logo appears.
Set Custom DNS
Often times, if your Internet Service Provider's Domain Name Servers (DNS) are having problems, you can switch to Google DNS or OpenDNS.
Please see http://www.iphonehacks.com/2014/08/change-dns-iphone-ipad.html for instructions.
Disable Wi-Fi Assist
It’s designed to allow iOS to seamlessly fall back from WiFi to cellular when the connected network isn’t capable of delivering sustainable or usable Internet.
To disable this, head to Settings > Cellular and flip the Wi-Fi Assist switch to the Off position.
Disable Wi-Fi Networking Services
This only relates to your location being used for Wi-Fi Networking, so it doesn’t disable Wi-Fi entirely.
Please see Settings > Privacy > Location Services > System Services
Reset Network Settings
Most networking related issues are fixed after resetting network settings.
Resetting these settings flushes caches and clears DHCP settings and other networking related info.
To reset network settings, open the Settings app and navigate to Settings > General > Reset and tap on Reset Network Settings