Its an easy way. Just click on this link to check whether your sim card is converted to 4g sim card or you have to get new 4g supported sim card.
Link : https://www.dialog.lk/4g-sim-request
If its not converted then you have to get a new sim card from franchise or authorize dealer because 4g supported sim cards are operating on specific frequency or some specific bandwidth.
At this time if you convert your sim card to 4g then you have a chance to win 5GB internet bundle.