I've also encountered this type of problem, All I do is to follow this steps:
Step 1: On your iPhone search and tap on “Settings” app.
Step 2: On the settings option, scroll down and select “General”.
Step 3: From general settings select “Reset”.
Step 4: After clicking the network settings, then select “Reset network settings” option. If you have a password for your device, you will have to enter it before you continue to the next step.
