I am still having issues with my phone: cannot make phone calls, send SMS and do not have internet. Basically, I have zero mobile network on my phone...
I've tried all your suggestions but I am still having issues.
1. Turn Airplane mode ON and then OFF: done
2. Turn your device OFF then back ON: done
3. Check here to make sure you have an active goodybag or top-up credit-: fine
4. Pop your post code into the service checker for local mast issues - https://giffgaff.com/coverage-and-service-fine
5. Perform a manual roam-not supported on my phone