Take a look at this Apple Support article:
https://support.apple.com/en-us/HT204051
Reset your network settings.
Settings - General - Reset - Reset Network Settings
Reset your device. This won't delete your data stored on the phone.
- Press and hold the Sleep/Wake button
- Press and hold on the Home button
- Keep holding both buttons until the display turns off and back on with Apple logo on it.
Alternatively, you may go to Settings - General - Reset - Reset All Settings