You may buy crypto on Cake DeFi directly with Fiat.
To do so, please click the "BUY BTC" 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 and ETH. Please note, depending on the payment service you select, there might be different cryptocurrencies shown and different procedures / requirements for the purchase.