Do you have 4G enabled?
Try it without if so.
If not, you may need a new sim card.
A previous issue such as this was resolved with a new sim card. Any O2 shop can help you with this.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.