Can you withdraw your crypto is what I'm asking. if you can withdraw or send your crypto to your personal wallet, then I don't see any harm in using them. But if they don't, then you should switch to a proper exchange instead. Sell your crypto to fiat, and use it to buy the coin you want in Coinbase for example, and then withdraw that to your personal wallet.
Sorry I'm not familiar with Fiat i dont know what it is. But My option in Revolut is only buy the crypto for euros , sell the cryptos for euros or send (maybe my crypto??) to a crypto wallet (it says add a crypto recipient / Transfer funds to an external crypto wallet)