This is likely due to APN settings. Please try the following :
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
Method 3
If the above 2 methods could not work, please try to reset the networking on your phone. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
For iPhone, Navigate: Settings > General > Reset > Reset Network Settings.
For Android phone such as Samsung, Navigate: Settings Settings icon > General management > Reset. Tap Reset network settings.
Reboot your device, when you are prompt to select Singtel (postpaid) or Singtel (prepaid), please select Singtel (postpaid)