How can I add a USB cellular connection in Linux (Ubuntu and Raspberry Pi OS)?

The experience can vary from modem to modem. Some modems will work automatically when plugged in, while others require special drivers or software to function.

Once the USB modem is working in Ubuntu, Speedify will detect it within seconds, and a new connection will appear in the Speedify user interface (if you are running it).

Once it appears, you can adjust the Connection Settings, such as Priority, to control how Speedify uses the connection.