iPhone stopped working using 3G/4G

I have an iPhone 6 Plus.

In the last week my phone has intermittently stopped working when I used cell phone data. This was whether I was on a 3G or 4G or LTE network. This seems to have been a particular problem when I used data heavy content. Sending images or voice messages via WhatsApp for instance.

I continue to receive notifications and am able to send iMessages most of the times.

But using websites on Safari or the Twitter app, Instagram app etc has only worked about 10% of the time.

When I'm connected via Wifi there are no problems.

I have double checked with my cellphone service provider and I have lots of data available.

I have also re-updated the latest version of the iPhone and iTunes and my Mac software.

I have rebooted my phone.

Are there anything else I can do as the phone is pretty useless if I can't use most of its functions using cellphone data?

Tks,