..Welcome to giffgaff and the community
You can check here to see if there is any mast problems in your area here
then If the phone signal is good and you are still experiencing problems you can try doing a manual roam.
Remember it’s updated hourly so may not of shown last time your checked
I would also try taking out your sim out and trying it in a friend / family members phone that is unlocked and working to establish weather or not it is the sim
I hope this helps;