Restoring an with 3uTools is generally not possible because Apple’s servers must verify and "sign" firmware before it can be installed. Without this digital signature, the installation will typically fail during the verification process.
If you found guides claiming 3uTools can restore any unsigned IPSW, those guides are either: how to restore unsigned ipsw with 3utools
: Download the exact firmware for your model from a trusted repository like IPSW.me. Restoring an with 3uTools is generally not possible
: You cannot use blobs from a different device; they must have been saved specifically for your device while that iOS version was still signed. how to restore unsigned ipsw with 3utools