Amex and Boa cards work fine online
(Amex safekey comes via email/phone when adding to grab, though Amex probably not as accepted everywhere)
https://www.flyertalk.com/forum/32811248-post5.html
Boa visa, verified byvisa works when online (eg buying tickets to Singapore tourist attractions)
Never had an issue with my chase Sapphire in person though (MRT station machine works, but didn't code as transit). Heard issues about using sapphire online in asia, since verifiedbyvisa doesn't work (didn't work for jurong bird park online, but boa worked)
try adding sapphire to Google pay/apple pay, and use that instead? Contactless is pretty much widespread in sg