I'm using Cake Wallet on GrapheneOS without any issues at all, buying Bitcoin and Monero.
AngWay "this currency pair isn't supported by any provider for the selected payment method please choose a different pair or try changing..."
This means what it says. It's not about your payment method or OS, it's that no companies support the combination of the cryptocurrency you're trying to buy and the fiat currency you want to convert from. If the specific combination you're trying to buy worked on your Samsung, it just means that payment providers have now dropped support for the currency combination.