With different networking options available—ethernet, Wi-Fi, iPhone tethering, etc.—you may find yourself in a situation where you want your Mac to automatically select one type of connection over another.
However, her internet connection is as slow as molasses, so I want to simultaneously use my iPhone’s tethering capability (via USB or Bluetooth, of course) to browse the internet.
macOS does let you prioritize network connections, so you can pick which adapter gets used first when your system tries to connect to local network and internet-connected resources.
Open the Network system preference pane.
Drag the items around in your preferred order.
This should retain the ability to access locally available resources, like the printer.