I apologize that your dealing with this issue. Have you tried to reset the network settings? If not, please follow the steps below. Also, try updating your PRL and Profile (steps below). Lastly, check if you are using VoLTE (Voice over data) this sometimes can create issues with sprint connectivity. If nothing works, please reach out to your carrier for further steps, perhaps you might need a signal refresh.
Go to Settings> General and Management> Reset> Reset Network Settings> Reset.
This action will reset all WiFi networks you previously have joined, bluetooth devices you have paired and any Mobile Data settings.
To update PRL and Profile:
Go to Settings> Scroll to Software Update > Update PRL> After this is done > Update Profile.
Hope this helps! Have a great day!