I would try a Manual reset, it usually works & is worth a try. This info. will help you.
***MANUAL RESET:*** https://community.giffgaff.com/t5/Tips-Guides/Manual-Roam/m-p/3345471#M32502
This is a helpful link when having trouble connecting to the internet at home or abroad:
If you can't get on the internet try this out on your phone: it's only the 'Generic settings', but worth a try.
1. Go to settings
2. Then go to mobile networks
3. Select a network other than O2(gg) i.e EE or Vodaphone
4. Come out of settings and it should say no network coverage or something similar
5. Repeat the process this time choose O2(gg) and this will update your number to the message centre and should help your txts
The message centre number in your message settings should be +447802002606