My iPhone keeps telling me my cellular data is off?
Apple has some information up at:


https://support.apple.com/en-us/HT201673


It basically points you to contact the vendor (i.e. your cellular carrier) to troubleshoot problems with cellular data. Apple would help you locate the cellular data on/off switch in settings, but once you've done that - you could back up your phone and erase it / restore a known good OS and re-test cellular data.

That would let you know it's not software or settings. Then you would need to determine if your phone needed a hardware repair (less likely but possible) or if your carrier can tell why the coverage is spotty or not continuos. They have the cell tower logs and know how your account is set up (as opposed to Apple that designed the device to work with hundreds of carriers).