My wife and I both have Samsung S23 Ultra phones on prepay with OneNZ

The phones were bought second hand in NZ, however her phone was originally bought in USA, so it shows AT&T when it boots up.

There is no problem with my phone, however she is having problems with hers.
She can make and receive txts and phone calls, but has no mobile data. That means that if she is out, she can’t use anything that needs mobile data e.g. Google and What’s App. Everything works ok if she is on WiFi.

I’m sure that it worked ok when she first got it a couple of months ago, but it doesn’t work now.

Have restarted, but no change. We did a factory reset when she first got the phone.

I tried a One NZ Shop and they couldn’t solve it. I tried Samsung online help and they said I would need to contact AT&T.

Any thoughts?