To fix that, go to Settings > Cellular > move the Cellular Data slider to on/green. If the slider was already set to on/green, try moving it to off/white, waiting a few seconds, and then putting it back to on/green. This might reset your connection and resolve the error. Remove and Re-Insert SIM Card.

Why can't I activate my mobile data?

Remove and Re-Insert Your SIM Card

Before rebooting, turn on Airplane Mode. Wait for 30 seconds, then turn Airplane Mode off. If you still don't have data, turn airplane mode back on, turn your phone off, wait for a minute, turn your phone back on, turn airplane mode off, wait for 30 seconds, then turn mobile data on.

Why is my cellular data network not available?

Update the phone system to fix software bugs

At this point, updating your phone system could fix your mobile network issue. Simply go to Settings > System > System Update, to check and install any pending update.