For the best user experience of GigSky's service on Apple SIM, always use the latest version of iOS and carrier settings. Currently, the latest version of iOS is iOS 12.1
To update your version of iOS:
https://support.apple.com/en-us/HT204204
To update your carrier settings:
https://support.apple.com/en-us/HT201270
(http://bit.ly/1MchsKk)