What if any pop up errors do you see when you try to connect to the Internet using Mobile Data @ChrisOT ?
Try a soft reboot by turning your phone Off and On.
Make sure Mobile Data is still turned On in your phone's settings please.
Is this happening in one particular area or everywhere ?
Check your network providers masts via any available service checkers.
Please do check your Apn settings.
Do you see 3G / 4G etc in your phone's status bar at the top ?
Try also going into Settings and search for Reset. Then reset your network connections.
Current Phone > Samsung Z Fold³ 5G.