Have you tried setting that 'minimum balance' option? According to the FAQ documentation, Binance should automatically convert the required amount of crypto each day to top-up the minimum balance on the card wallet.
This option allows to set minimum balance in USD and top-up as soon as your balance is below that minimum. But it does not convert crypto to fiat. Even in Binance dashboard the balance of your card is shown in USD, but when you enter your card you see different cryptocurrencies there.
I dont use Binance card as my primary card, just keep some balance there. Maybe in future, when I plan to get some fiat from crypto deal, I will try one more time this automatic conversion opportunity. But so far I plan just to trade and hold.