Signal issues can usually be narrowed down to a phone, SIM or local mast problem. Please try the following and report back:
• Network / mast issue
Check if there are any reported issues in your area, and try to verify with other local O2 / giffgaff users around you if they are experiencing similar problems: https://giffgaff.com/support/sim-and-number/change-your-sim
. Your existing phone 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.