IOS – Mobile data keeps switching off

A majority of the time the issue is with a random app or service running in the background. Less often it's because the carrier has to update their software on the device. In both cases, turning off cellular data would solve this issue.

Occasionally there is a problem with the phone itself. Go to Settings>Cellular and note how much cellular data is being used. Then turn off Cellular Data. After two or three hours check if cellular data usage has changed. If it has, restore the iPhone, and if necessary replace it if you have the option.

If there is no change in the cellular data usage I would contact your carrier and let them know.

This is probably not a problem with your phone. It is up to the carrier to decide which individual handsets are actually allowed on LTE on the current cell tower. So you might just be experiencing that your carrier hasn't got enough capacity to allow everyone on LTE, so they put you on a slower network.

So even though a coverage map tells you that LTE is available - it doesn't necessarily mean it is available to everyone.