Q: iMessage on my 6s plus keeps turning off my phone is telling me to turn on cell data but it's on

Can you go online, using cellular data, for example open a page in Safari?

Does iMessage work if you are connected to Wi-Fi?

Did you already try to force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again? You will not lose data or settings, but it can cure some glitches.

If this does not work, set it back to factory settings again, this time without using the backup, set up your personal data manually and  test the cellular data connectivity.

Use iTunes to restore your iOS device to factory settings - Apple Support

Where you able to register for iMessage before this started to happen? Did iMessage ever work? Can you send normal text messages?

Did you already check for carrier updates, to make sure that you can use cellular data?

Does your carrier support iMessage and FaceTime?

iOS: Updating your carrier settings - Apple Support

If you get an error when trying to activate iMessage or FaceTime - Apple Support

About cellular data networks - Apple Support

What happens if you disable iMessage in Settings/Messages, restart the phone and enable it again?

Did you make sure that your phone number is selected under send&receive in Settings/Messages?

If iMessage stops working after sending a few messages successfully, and setting it up as new device, without using any backup data or installing 3rd party apps, did not solve that issue, take it to an Apple Store or Apple Authorized Service Provider and let them check on it.

You can also contact Apple Support about that by phone.

Find an Apple Authorized Service Provider

iPhone - Contact Support - Apple Support