You may buy crypto on Cake DeFi directly with Fiat.
To do so, please click on the "BUY CRYPTO" button on the top right of your Cake DeFi page. Then this pop-up window should appear:
Under "I want to buy" you can choose DFI, BTC, ETH, EUROC and SOL. Please note, depending on the payment service you select, there might be different cryptocurrencies shown and different procedures / requirements for the purchase.