Thanks Kaspars, that's the best case. No cash needed when using Grab with saved credit or debit card.

In the meantime I have found some information that sounds like a good backup plan:

In Cambodia an app called "PassApp" seems to be used for rides as well. To sign up you need a Cambodian number, which is easily available because local SIM cards are extremely cheap, either for free or for just a few USD at the airport or mobile phone carriers shops like "Smart".
Here is an overview of the networks / SIM card options in Cambodia: https://prepaid-data-sim-card.fandom.com/wiki/Cambodia