Mobile banking app

It is very possible that your bank is now using an app that will no longer work with the older iOS version that is running on your phone.

If that is the case, you basically have two choices:

1. Upgrade to a newer iPhone that can run iOS 13+, or
2. Restore your iPhone from a backup that still included the older version of this app.

What do you mean exactly by "not working?" Has it worked before? Did you update the iOS version on your iPhone? Is this phone still running an older iOS 10.x version?