You don’t always need an external check link. You can find your SEID natively in iOS:
Are you checking the SEID to or to verify the authenticity of a used phone? iPhone iOS 18: How to Find The SEID Number seid iphone check link
If you need to your iPhone’s SEID (e.g., for carrier support), follow these steps – no external link required: You don’t always need an external check link