Your iPhone was not hacked. Your bank account can be hacked without an iPhone by anyone who knows your bank account number, which is almost certainly available on the dark web. If you have ever written a check, or had your bank pay with a check initiated from online banking that is an easy way to get your account number. Or if you ever used a debit card at a public ATM, gas pump, or any other merchant that takes card swipes or dips your account can be compromised that way also.
If it makes your bank happy you can restore your phone to factory settings and not restore a backup, which might be easier than convincing them that your phone was not hacked.
Restore your iOS device to factory settings - Apple Support