my iphone 5 won't use mobile data when wifi not available

Hello there, mamadoos.

When troubleshooting issues with cellular data connection issues, the following Knowledge Base article offers up some great and practical steps for resolving the issue:

iPhone cellular data connection issues - Apple Support

Follow these steps for help with cellular data issues on your iPhone. After performing each step, test to see if your iPhone still has the issue.

1. Toggle airplane mode: Tap Settings, turn airplane mode on, wait three seconds, and then turn off again.
2. Restart your iPhone.
3. Make sure that your software is up to date:
   • Check for a Carrier Settings Update: Tap Settings > General > About.
   • Check for an iOS Software Update: Tap Settings > General > Software Update.
     Some updates may require a Wi-Fi connection.
4. Remove the SIM Card and reinsert it. Allow the iPhone to acquire the network again.
5. If your SIM card has SIM PIN enabled, try turning it off: Tap Settings > Phone > SIM PIN.
6. Try another location. If a different location works, but the original location still doesn't, contact your carrier to report the issue.
7. Reset network settings: Tap Settings > General > Reset > Reset Network Settings.
   Resetting network settings on your device will erase all current saved settings, including Wi-Fi passwords, preferred networks, and VPN settings.
8. Restore the iPhone as new.
9. Contact your carrier to:
   • Verify that the iPhone is properly set up on the account with the appropriate, current data plan.
   • Verify that there are no account-related blocks.
   • Find out if there are specific error messages in the carrier logs that could help determine why the issue is occurring.

If none of the above steps resolves the issue, contact your carrier, make an appointment at an Apple Retail Store, or contact AppleCare.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro.