iPhone has 5G connection but apps only work on wifi

friend of mine previously had an iPhone XR & upgraded to a 15 plus . the XR would connect to wifi easily/normally & every app would work from playing music to scrolling the internet to using maps but as soon as it was disconnected from wifi everything stopped working . he could still receive calls & texts but no maps for directions , music for in the car or at work , nothing . cellular data is turned on , no crazy vpn or anything was set up , wifi calling was off , nothing could fix it & AT&T & Apple employees were stumped . upgraded to the 15 plus thinking it could fix the issue , but no luck . i'm stumped .