Its likely due to APN settings.
Method 1
Reboot your device, when you are prompt to select Singtel (postpaid) or Singtel (prepaid), please select Singtel (postpaid)
Method 2
Go to your APN settings, set
APN Profile : Singtel(Postpaid)
APN Name : e-ideas
Username/Password : 65ideas
Android
Go to Settings > Connections > Mobile networks > Access Point Name
iPhone
Select Settings > Mobile Data > Mobile Data Network