Can I turn off IPv6 and just use IPv4 on my iOS device?
On a Mac computer, download Apple Configurator from the Mac App 
Store
In Configurator, go to File > New Profile (or Command+n)
Fill out the General section, then go to Cellular and select Configure
Under Configured APN Type, select Default and Data APNs
Fill out the necessary data using your carrier's APN settings
Under Data APN Supported IP Versions, select IPv4
Sign and flash the profile to your phone