The most recent version of iOS 9 is now almost 3 years old. Apple will no longer release new iOS 9 versions, which means that security leaks discovered in the last 3 years will never be resolved for your version. And of course you miss all the new features of iOS 10 and newer, including new developer APIs that apps use.
If you want to use the latest version of the bunq app, then you will really have to upgrade.
Incidentally, pretty clever that your iPad (it must be a 2nd or 3rd generation if iOS 9 is the latest version) has lasted 6 years!