Have you checked coverage & service in your area?
You can check the coverage & service in your area by using following linkhttps://giffgaff.com/coverage-and-service
Please try manual roam settings
Go to your phones "settings".
Select mobile network options.
Change the Network Selection Mode from Automatic to Manual
After a few moments a list of available networks will be displayed.
Select any other network that is not your home provider network.
After a few moments you will see "No Service" on the display.
Now select your home provider(which is o2) network and change the selection back to Automatic.