Hello.
Please try these below to fix your connection:
- Reboot your phone: Press and hold the power button for until the phone turns off.
- Update WhatsApp to the latest version.
- Turn off airplane mode.
- Open your phone's Settings app > Data usage> and make sure that Cellular data is turned on.
- Make sure background data is not restricted Go to settings app > Apps> WhatsApp> Data usage.
- Contact your mobile provider and make sure your APN settings are configured correctly.
- Upgrade your Android OS to the latest version available for your device.