make sure 4g Carrier available on your location..
- Make sure that your device is connected to a Wi-Fi or cellular network.
- Tap Settings > General > About. If an update is available, you'll see an option to update your carrier settings.
- To see the version of carrier settings on your device, tap Settings > General > About and look next to Carrier.
If you insert a new SIM card into your iPhone or iPad, you need to download the carrier settings for your new carrier.
for more details:If you see No Service or Searching on your iPhone or iPad - Apple Support