Well, it entirely depends on your bank, some banks intentionally block anything that’s not Google certified Android and there’s not much you can do about that.
Edit: here’s a forum thread with user reported bank apps, not everything is there, but you might be lucky: https://forum.sailfishos.org/t/banking-apps-on-sailfish-os/18438
If you want, tell me your bank app and I can at least try installing and opening it.
Only if you buy his PR.