So do you have 'no network coverage', or 'full signal'?
If you have full signal bars, then basic calls / texts should work.
Please check / try the following and report back in detail:
• Can you text normal mobile numbers?
• Network / mast issue
Try to verify with other local O2 / giffgaff users around you if they are experiencing similar problems, as signal checkers are only guides
• SIM issue
Test your giffgaff SIM in another unlocked phone. If the same SIM does not work in multiple devices, then you may have a faulty SIM. You can order a new SIM or find a spare inactivated one, and do a ‘SIM swap’: https://giffgaff.com/support/sim-and-number/change-your-sim
. Your number / goodybag / credit get transferred to the new SIM so you don't lose anything.
• Phone issue
Test different known working SIMs in your phone. If all SIMs get no signal in the same device, then there could be a potential problem with your handset.