Hello magpie99,
Thanks for using Apple Support Communities.
In order to resolve this issue where you're unable to activate your iPad with cellular data, please follow the steps below. Please note that even though the article states it is for an iPhone, the same applies to your iPad.
- If your carrier provided a SIM card, make sure it's in your iPhone.
- Restart the iPhone.
- Try to reach the activation server another way, then try to activate.
- Try connecting to a known-good Wi-Fi network if you can't activate using a cellular data connection.
- Try connecting to iTunes if you can't activate using Wi-Fi.
- Restore the iPhone.
- Check the system status for iOS Device Activation. If the iOS Device Activation status isn't green, try activating your iPhone later.
If you receive an alert message when you try to activate your iPhone, place the iPhone in recovery mode and perform a restore. If you still can't complete the setup assistant due to an activation error, contact Apple for assistance.
If you're asked to enter a password to proceed with activating the iPhone, Activation Lock might be enabled as part of Find my iPhone.
Get help activating your iPhone - Apple Support
Take care,
Alex H.