If this is the case, you will need to find a third cryptocurrency that can be traded for SOL on the exchange. In most cases, you will be able to convert or trade your fiat currency to a stablecoin (USDT, USDC, BUSD, etc.) and use this stablecoin to buy SOL on the exchange. This will not be an issue whatsoever, it only mean you'll have to make one more trade.