It's definitely the giffgaff sim I have checked it twice. The giffgaff is shown in the list, check the 7th message for more details
I'd get another blank Sim and do a Sim-Swap.This has happened before when people come back from the EU.I suspect the Sim-card was weak or poor Quality Control.
I'm too impatient to wait for a free Sim so..
buy one for less than a pound at B&M/Poundshop/Tesco/Argos/Iceland and even my Corner shop sells them now and do a,"Sim Swap".Do NOT activate the New Sim e.g put any money on it
is as a Blank unused Sim is needed for a Sim swap and the Sim-Swap process will activate the new Sim.
A lot of these shops are open Sundays until 4pm and 8pm weekdays.
and here's an easy to follow video,
you can only do a SIM swap between 4.30am and 9.30pm and it can be like when you first join GiffGaff e.g shut the phone down for a few mins and or keep restarting the phone over a few hours after the Sim Swap.
This will transfer your number and allowances across.