Why is my iPhone 13 Pro Max SIM internet not working?

Try this

Restart your device. If you're not sure how to do this, follow these steps to restart your iPhone or restart your iPad.

Contact your carrier 

Issues with your carrier or account might affect your service. Contact your carrier to:

• Verify that your account is active and in good standing.
• Make sure that you're in an area with cellular network coverage and there are no outages in your area.
• Check that your device isn't blocked or barred from receiving cellular services and is set up with the right data plan.
• Ask if your local government requires IMEI registration to use an iPhone that you originally purchased outside your country or region.

Only your wireless carrier can access and manage details about your account:

• iPhone wireless carriers
• iPad wireless carriers

Update your iPhone or iPad

Update your iPhone or iPad to the latest version of iOS or iPadOS.

Check for a Carrier Settings update

To manually check for and install a carrier settings update:

1. Make sure that your device is connected to Wi-Fi.
2. Go to Settings > General > About. If an update is available, you'll see an option to update your carrier settings.
3. To see the version of carrier settings on your device, go to 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.

If you see SOS, No Service, or Searching on your iPhone or iPad - Apple Support