Welcome to the F-Secure Community. Thank you for your post.
Regarding the USB port on the F-Secure Sense Router, USB tethering is not supported. The USB port on the F-Secure SENSE router serves different purposes, such as being a network connection (using protocols like Ethernet-over-USB and PPP-over-USB), a device console (serial-over-USB), an attachment point for a printer (if the device has an embedded print server), an attachment point for a USB storage device (if the device has an embedded storage server), and a means for administrative updates to the firmware, configuration, or other stored information.
If you have any further questions or concerns, please don't hesitate to let us know.
Thank you and have a great day!