Have you had a new sim through the post @Pussinboots ?
It might be you need to swap, or ask for a new one if you didn't get one. Best do this in a shop, with photo ID, as calling for a new one will get one sent out but also your current one turned off!
Also try making sure the data settings are ok - they should match the below:
https://community.o2.co.uk/t5/Android-Devices-Samsung-HTC-Sony/Connecting-Android-Phone-to-Data-Serv...
https://community.o2.co.uk/t5/Apple-iOS-Devices-iPhone-iPad/Connecting-iPhone-to-Data-Services/m-p/5...
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