When connecting your bank and its sub-accounts to QuickBooks Online, you'll have to consider how it shares your posted transactions. You can connect the parent account if the transactions download into one account. However, if the transactions were downloaded to the individual accounts, you'll need to connect the sub-accounts. I recommend speaking with your accountant about this to make sure what would be best for your business setup. Here are a few articles that explain credit card sub-accounts in QuickBooks Online:
After deciding what's best for your company and getting the transactions imported, then you can reconcile the accounts. This is the process of matching transactions entered into QuickBooks Online against your credit card statements.
I'm providing a few helpful links to help you learn and grow with the program! You'll be a pro in no time.