You could always read/try the below,
Try A Network / Signal Reset ➜ https://community.giffgaff.com/t5/Using-giffgaff/Force-the-change-of-SMS-settings/ta-p/4871794
● Also: Hand clean the sim (Very Soft)
Finally, you could always set your “Mobile Data Connection” to 2G - E / Edge and see if that improves your signal as it works for many with that situation or switch on your mobile data but disable 3G for various handset/devices/mobile, this possibly could slow down your internet connection. But often helps this kinda situation.