First, separately from the PX4 Support Package, you have to ensure that the Telemetry radios are properly set up. Below are some of the PX4 documentation links that you can refer to:
With using the PX4 Support package, you will have to enable mavlink on telem Port in 'rc.txt'. Refer to the second note in this documentation ://www.tianjin-qmedu.com/help/releases/R2020a/supportpkg/px4/ug/enable-mavlink-px4.html
Note that the PX4 support package supports serial communication via TELEM ports. Refer this link (//www.tianjin-qmedu.com/help/releases/R2020a/supportpkg/px4/ref/port-mapping-for-serial.html) on how to set the serial port for Telem ports. This has been tested via physical connection. That is, 'Pixhawk Telem ports' -> 'FTDI' -> 'USB' -> 'PC'. Refer to this link (//www.tianjin-qmedu.com/help/releases/R2020a/supportpkg/px4/ug/external-mode-px4.html) to refer the physical connections.
