If you can receive SMS from DNC website (
https://www.dnc.gov.sg/) on your existing ported-in number (not the temporary number), and you can't access mobile data, and cannot make / receive calls, it means that the porting-in succeeded, but you are only on 3G. Why?
This is because your retailer screwed up the porting-in process. The retailer only did the porting in registration for you, but DID NOT TOP UP the $10 data plan for you. So, it means that your heya line is in a "limbo" state; technically, it is between the 60-day and 90-day expiry for those who are interested.
You need to go back to your retailer and ask him to top up $10 to your existing ported-in number, NOT the temporary number. This will provision / activate your line properly.
The proper process for the retailer in the first place is to do the porting in registration for you, take a picture of your temporary number, and immediately top up $10 to your temporary number. One visit should suffice.
Now you need a second visit. If the porting in succeeded, the temporary number is now dead, and the top up of $10 data plan should go to your existing ported-in number.
Once this is done (the retailer should not charge you extra because he already collected $10 from you during your first visit), you should get mobile data and be able to make/receive calls and have 4G voLTE reception within five minutes of the top-up.
Please let us know how you get on.