Sim

[Nell1]

I don’t seem to have any mobile data, in settings it’s just saying set up mobile service

[TallTrees]

Hi @Nell1 

Welcome to the O2 community 

You didn't say what mobile phone you have

I have a Samsung and I go into my drop down 

List and press a box with mobile data written on it. 

When I leave my home WiFi it connects to the Internet.

[MI5]

@Nell1 

Check guides Guide: How can I sort out my network issues? 

https://www.o2.co.uk/apps/my-network

Guide: How to find help & contact O2