Unable to Activate

My brother upgraded to the iPhone 15, so I got his iPhone 13. So he reset it, but when I tried setting it up, I received an error message saying, "Unable to activate, an update is required to activate your phone." I've tried everything: putting it in recovery mode, updating, and restoring it twice. I've called Apple three times, but they weren't able to fix it. The last representative told me to make an appointment with Apple, which I will do, but I think they will charge me a lot. Does anyone know anything about this? The phone was working perfectly just one week ago.