Three years ago Mustafa Ismael launched Karcsham Co., a Kenya-based company that resells Starlink devices and manages subscriptions for thousands of customers across a dozen African and Latin American countries. At first he used his local bank card to pay the monthly bills. But the 5% foreign exchange fees soon began eating into his profits. Then came Rizon, a Delaware-based neobank, that lets users pay through stablecoins, cryptocurrencies typically pegged to the US dollar. Now, with a few taps...