Hello,
If your bank website doesn't offer any type of file download that will work with QuickBooks Online, you can setup your bank account in your chart of accounts.
Go to Gear Icon/Your Company/Chart of Accounts
Click the green New button in the upper right and select Bank as the account type. (picture attached)
Fill out rest of information and click Save and you'll have your account setup where you can manually enter transactions that affect the account.