iPhone 7plus for bank transactions

if the bank you are using made an app with higher ios requirements than your phone supports then your options are

1. get a new phone that meets the requirements
2. change bank to a bank which app has lower ios requirements
3. contact the bank / app developers to inquire about your options How to contact an app developer - Apple Support

You can update an iPhone 7 Plus to the latest release of iOS 15. You cannot update an iPhone 7 Plus to iOS 16 or higher. That means that your iPhone 7 Plus cannot run apps that require iOS 17 or higher. If you want to run such apps, you will need to get a newer iPhone.