Correct on Grab, at least for me. I use Grab in SEA and haven't had any trouble with a US phone number. However, I can't even open the app when in the US. (I fear I didn't properly check out of a ride in Jakarta a few months ago, but couldn't do anything about it a day later.)

I see people suggesting car rentals in Langkawi. I'd probably do that on a future trip. I also wonder how prominent/safe bicycling is there.