If your iPad or iPhone has no service you should follow these steps.
Firstly, you should check your coverage area.
Make sure that you are in an area with cellular network coverage. Later, to turn cellular data on or off, go to Settings and tap on Cellular or Mobile Data.
If you are traveling internationally, make sure that your device is set up for data roaming. You have to fo to Settings, hit Cellular, then Cellular Data Options and lastly Data Roaming.
Furthermore, you can restart your iPad or iPhone if there is no service.
To manually check for and install a carrier settings update:
Additionally, if you insert a new SIM card into your iPhone or iPad, you need to download the carrier settings for your new carrier.
Another way is to take out the SIM card.
After removing the SIM card, put it back in. You might need to contact your carrier:
You can also update your network settings.
This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before.
Moreover, update your iPhone or iPad with the latest version of iOS.
If none of this works, contact your carrier. Issues with your carrier or account might affect your service.
Contact your carrier to:
If your carrier confirms that there is no issue with your account or network, but you still cannot connect to a cellular network, contact Apple Support.