iPhone 'Could not activate mobile data network' after iOS 11 update - FIX

I have been struggling with iPhone not connecting to Mobile/Cellular Data services even though the LTE/4G icon is showing normally. This seems to happen after every major iOS update. Most recently from iOS 10 to iOS 11 and now with 11.3 too.

It brings up this alert 'Could not activate mobile data network' and you have to restart your iPhone to get it working.

I finally discovered a fix - I hope it works for you.

Firstly, ensure you have the latest Carrier Update (with WiFi connected open Settings>General>About) and accept a carrier update if one appears on screen.

Next, turn off your iPhone.

Remove Sim Card.

Turn on iPhone and connect with WiFi.

Go into Settings>General>Reset and Reset Network Settings.

Turn the phone OFF.

Insert Sim Card.

Turn the phone ON.

That worked for me after trying EVERYTHING including a full restore via iTunes which is a long process.

Good luck.