Is supplication alone sufficient to repay an interest-bearing debt after transferring to an Islamic branch, given that there is no other way to repay it?

You have done well by repenting from usury and supplicating to Allah to relieve your distress. Among the etiquettes of supplication are not to hasten, to be certain of the answer, to eat lawful food, to abandon sins, to be mindful of Allah’s Names and Attributes, the vastness of His generosity and bounty, the stories of the prophets, to perform good deeds and avoid evil, and to observe the conditions of supplication such as having a present heart and seeking out blessed times. No matter how long the wait, Allah's relief is near. When you supplicate, Allah will either grant you what you asked for, or avert evil from you, or store up reward for you as good deeds on the Day of Judgment.