I was able to resolve this issue in my particular situation and it (strangely) may have been due to my phone APN configuration. I have my iPhone 6 on FreedomPop and had this issue and it bugged me like no other. I simply went to the FreedomPop app, went to the settings (or My Account), tapped on APN Configuration, re-created the profile by filling out the dropdowns and installed the APN profile. Once you hit the create APN button there, you will have to press the Accept or Install button (I don't remember exactly what the message said). That prompt can go away pretty quickly, so you might need to run it again. Once it installed, I rebooted, and all was fine in the world again.
I'm not saying that this is THE solution, but it certainly worked in my situation. Maybe it's linked to APN configuration.
iPhone 6 on iOS 10.3.3