With the app on your phone, all you need is internet access. It doesn't matter what SIM is driving things. So you'll be home and dry if you have an Malaysian SIM installed.

I can't remember if Penang airport is without free internet, or if it was simply down on my visit some months' back. I piggy-backed the internet of a burger-bar to summon Grab. With your local SIM you avoid such clowning about.

As for the price of your Grab trip, you should be able to use the app to check. I just did just that, and it comes up with MYR48 for a bog standard car, up to MYR100 for the luxury transport you undoubtedly deserve.