@Geanina Try to change your apn to mobile.o2.co.uk and see if that makes any difference. If it doesn't, you'll need to contact customer service. https://www.o2.co.uk/contactus
We're gonna need a bit more from you please?
Network issues? Guide: Is the network down for me or everyone?
Sim issues? Guide: Sim Swap: a mini guide 2017 update
I'm guessing overseas so see the advice by @Bambino
Based on the info given, we can do nothing but guess 
@Geanina can you give us some more detail on your issue as requested above so we can make sure you're getting the right advice. 
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