This Knowledge Base article will show you how to set up your IOS device in order to use OpenDNS.

Also, this works the same on all iOS devices.
Changing your IOS device DNS settings:
- From the IOS device home screen, tap Settings.
- Tap Wi-Fi, ensure it is enabled and your wireless network is connected.
- Click the
symbol next to your wireless network, as shown below.
- The screen shown below appears. Tap the Configure DNS field.
- Ensure Manual is selected and delete the current DNS servers by tapping on the
symbol.
-
Tap Add Server and enter OpenDNS resolvers 208.67.222.222. Repeat this process to add another DNS server as follows 208.67.220.220, as shown below.
- Tap Save to exit the menu.
That’s it! You’ve updated your IOS device DNS servers!
source:
https://support.opendns.com/hc/en-us/articles/228008947-IOS-11-Configuration-for-OpenDNS