The thing with the Grab is that for some reason won't let you add non Asian credit card unless you are in the region. I had the same thing happened few years ago. Downloaded the app in US but wasn't able to add the card. Once I landed in Bangkok I had no problem adding the same card to the account.
Last year I had to change credit card on file (expired) but again I wasn't able to do it while in US. Once I was in Thailand I had no problem adding the card.