Best answer by terrywebbs
Firstly try turning your mobile off and back on again this can resolve 80% of issues.
This link is the official Coverage Checker: https://www.idmobile.co.uk/help-and-advice/uk-coverage-checker
That should keep you up to date on any ongoing or known issues in your area. If you have in fact done that and it's stating some problems were only recently fixed, then it could be that things haven't completely gone back to normal yet after any such work.
Where you receiving decent service when you first received the SIM? If so, we should expect it won't be too long until things are all back to normal and will continue that way from there.
If problems persist beyond a further 24 hours, please get in touch with our Social Media Team over at:
They'll be happy to monitor things and troubleshoot things further from there if needed.