Astrill Setup Manual:How to change your iPhone's DNS
From the iPhone's home screen, tap Settings.
Find your wireless network in the list, and then tap the "i" icon.
Tap the DNS field.
Delete the current DNS servers, and enter the new DNS servers. (If you enter more than one DNS server, be sure sure to separate the servers with commas.)
For Google DNS- enter 220.127.116.11 and/or 18.104.22.168