Unable to use TELEM2 on CUAV V6X

PX4 Autopilot
1

V6X has several serial port Serial Port Mapping

So far, I tried TELEM3, UART4, they all worked for mavlink or ros2 communication.

But TELEM2 not works.

By param show -a

x   SER_TEL1_BAUD [853,1424] : 57600
    SER_TEL2_BAUD [-1,1425] : 921600
    SER_TEL3_BAUD [-1,1426] : 57600

the default SER_TEL2_BAUD is 921600. So start mavlink at default baud rate

mavlink start -d  /dev/ttyS4

it not works.

Change baud rate to 57600, then start mavlink on

param set-default SER_TEL2_BAUD 57600

reboot 

param set  SER_TEL2_BAUD 57600

mavlink start -d  /dev/ttyS4

still not works.

Just can not figure out what’s the different between TELEM2 and other UART like TELEM1 or TELEM3