Manually re-install apn settings:
Download the giffgaff app this should help with the correct settings, depending what phone you are using:
Check the signal strength on the phone screen. If you generally get good coverage in this area, check if there is no temorary mast issues:
Or network issues:
Remove any accessory covers or shells from the phone temporary to check if covers affect the phones's signal strength.
Try changing the network connection to 3G ("Settings" > "Cellular (mobile network)" > "Highest connection speed").
Try reconnecting by switching "Flight mode" On and back Off ("Settings" > "Flight mode"). This resets the active connection.
Switch off your phone and take out the simcard. If the simcard is not properly inserted or if the simcard tray is not fully closed, the phone may get disconnected from the network.
Try your simcard in another phone to check if the issue is with your phone or your simcard.
If you continue to have problems then ask an agent, they can look into this for you: