Cellular transfer

Hi so i just got a new iPhone 14 Plus for my mom , she had an iPhone 6s , of course everything was transferred but the only thing that didn’t transfer was her cellular data , I’ve tried the SIM card trick , went on the company website to try and activate But couldn’t. I had an old XR that I put the 6s SIM card in & It still wouldn’t transfer. I don’t know what to do next , any tips.