Allow me an opportunity to help you configure your printer settings in a way that works best for your business. Whenever you press Print in QuickBooks Online, the software prepares the information for printing and hands it over to your internet browser to pass on to the physical printer. This means that QBO plays no role in deciding the default device for all printing tasks, and rather this setting must be configured either in the browser or the operating system you're using.
As a brief example, here's how to switch your default printer using Google Chrome:
Change default printer using Chrome
Navigate to the page or information within QuickBooks you wish to print, and click the Print button.
From the Print Preview that appears, select Print. In the field next to Destination, click Change.
Under the Print Destinations section, you'll see all available printing device on your PC/network. Select Manage at the top of the list.
A Google Chrome settings page will open directly to the Printing section. Click Printers ➤.
A Windows Explorer window will appear displaying all printing devices on your PC/network. Right-click any of these devices and select Set as Default Printer.