Having issues with the data is never fun but I can help you narrow in on the what is causing it. One of the first thing we need rule out is a known or reported mast issue(service outage) . The checker should give you details of any local issues, including planned work, as well as showing signal strength (2G, 3G & 4G) & outages.
There are two ways to identify a service outage:
❇️ An outage is identified in your area by the mast service checker here http://community.giffgaff.com/t5/Using-giffgaff/Force-the-change-of-SMS-settings/ta-p/4871794
knowlede base link on how to perform the manual roam.
Finally you can try turning off your device for a couple of minutes and back on again to see if that helps.