It seems to me that you are either in an area with poor reception for giffgaff (o2), or there's an issue with the local mast.
If you check here you can see if any issues are reported. https://giffgaff.com/coverage-and-service
I'm sorry your experience has been less than ideal.
Providing the make and model of your dongle might help someone provide advise, but, seeing as you have managed to use it I'm guessing it is set up correctly.
One thing I can suggest is trying different places in the house, sometimes something as simple as being in a different room can make a dramatic difference.
Sorry I couldn't be of more help.
Edit to add.
The speed you quote as what you have been getting in your other thread suggests to me that you have only been able to connect to a 2G signal. This would indicate to me that there is a mast/signal issue.