Black-listed iPhone 12

Looking for some advice,

In February 2021 I brought an iPhone 12 straight from Apple, I lost my phone in Italy in June 2022, my mum reported it as lost/stolen to O2, so my iPhone was locked. We then retrieved it again later that night, and my mum reported it as found.

However when I got back to England the signal was still not working, and we were in constant contact with O2 to try and fix the signal, we tried new SIM cards and a SIM card from giff-gaff, resetting the phone (from their end and my end), and finally they said I will have to get in touch with Apple because they're unable to fix it.

I went to an Apple Store to try and get support, and they said I need to call Apple Support, which I did, and the woman said that the phone has been black-listed, and that there is nothing they can do about it, O2 are the ones who have to take it off of that list.

I just want to see/know if there is an easy way to fix this?