It's a chicken and egg story.
Singapore population, technology oriented, could afford the devices because of the GDP/capita. Therefore, Singtel and Starhub, have invested heavily in 5G. Especially Singtel.
Also, it was easy for them to reach 95% national coverage in just one year because of the geography.
As soon as they reached that coverage, it was legit for them to charge a premium for 5G. But that premium is symbolic for prepaid, in an otherwise very competitive and very inexpensive market.
DTAC don't have these advantages of a GDP/capita (10x less than Singapore) or a smaller geography.
For mobile users today, the 5G vs 4G experience is unnoticeable because most 5G networks are non standalone (they rely on the legacy 4G packet core to carry traffic).
For operators having invested in 5G standalone (with a totally new much faster core), then 5G can hold its promises, in particular because of edge application use cases.
But 5G standalone penetration in operators is slow in most parts of the world (except China, and USA).