If you have performed a sim swap then try turning the phone off and on to help reset the connection.
It can take up to 24 hours sometimes to fully settle.
Ensure you don't have airplane mode turned on which stops connection and ensure your mobile data setting is turned on on the phone.
put your full postcode in the service status to ensure your masts are ok too.
http://status.o2.co.ukCheck with O2 on your account via 202 contract or 4445 Payg.
Also send ACTIVE to 2020, save the settings and Reboot your phone.
Finally it has been known to be supplied with a faulty sim card so it could be worth visiting an O2 shop to be supplied with another.
