Have you checked your network status? https://status.o2.co.uk/
Community help here Guide: How can I sort out my network issues?
Or you may need a new sim. You can get one by calling O2 on any number in this guide
Hope that helps and welcome to the forum
Veritas Numquam Perit
Also check if you have 5g enabled in your settings (if you have a 5g phone).
If so, turn it to 4g/3g/2g as it may be stumbling trying to find a 5g signal.
Have you checked your local network @Ashleighgodwin ?