You're able to add a bank account even without connecting it to the online banking feature in QuickBooks Online (QBO). It's possible through the Chart of Accounts page and when you record your transactions.
If you choose to do it through the Chart of Accounts page, kindly refer to the following steps:
Go to the Accounting menu.
Select Chart of Accounts.
Click the New button.
Fill in all the necessary details (i.e. Account Type, Detail Type, and Name) of your account.
Select Save and Close.
I've attached screenshots below for your reference.
Also, you can create subaccounts to keep your Chart of Accounts organized. This way, you're able to breakdown your income, expenses, and get a more detailed report. For the step-by-step guide, you can check out this article: Create subaccounts in your chart of accounts in QuickBooks Online. It also includes instructions about changing an existing one into a subaccount.
Additionally, I'd recommend pulling up the Account List report. This way, you're able to view the complete list of your accounts in your Chart of Accounts and their balances. Just go to the For my accountant section from the Reports menu's Standard tab.
Please know that you're always welcome to comment below if you have other concerns or follow up inquiries about managing bank accounts and monitoring transactions in QBO. I'm just around to help. Take care.