Why does my iPhone work on Wi-Fi but not cellular data?

My phone works well while on wifi but once i am off wifi it stops working and won’t properly connect to my cellular data!! it so frustrating and i thought it was a phone issue so i went from an 11-15+ and i am having the same issue on the new phone. i have been to apple 2x and my provider 3x’s. No one else on the family plan is having an issue, so i’m not sure if it’s the cellular network or my phone. A couple people have posted with a similar issue and i wanted to know if anyone found a solution! someone please help me.

[Re-Titled by Moderator]