If you could also try turning your device OFF and take your sim card out, then wait 5 minutes,and turn your device ON again,as this can help in most cases. Also try turning your airplane mode ON and OFF again. If the signal checker is showing NO reported issues,can you also perform a manual roam, which basically is searching for networks on your device ,see here http://community.giffgaff.com/t5/Using-giffgaff/Force-the-change-of-SMS-settings/ta-p/4871794
if your on 4g switch to 3g if your on 3g switch to 2g then turn your phone on and back on then try switching back leave a couple of mins when turning phone on/off