Urgently needed banking app keeps crashing

Try this, If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support and contact the app developer if that does not work. See this for contacting App developers. How to contact an app developer - Apple Support

Unfortunately, that is going to make a difference. When they provide you with things like this, they have to support it. Someone else cannot provide support for an individual or company's app. Keep trying to contact them.