iPhone XR “unable to activate”

How do I fix my phone? Unable to activate showing on my phone after a hard reset. Reason for the reset is it had error showing in my cellular data. The guy at the At&T store reset it Also tried hooking it to my computer and Through iTunes and on iTunes it says (iPhone could not be activated because the activation information could not be obtained from device. the guy at the store also put a new SIM card in my phone trying to fix it. HELP!