Where is the debug serial port of HLK-RM28D?

I got a HLK-RM28D-KIT-V1.0 board, it come with Hi-Link's firmware by default.

I would like to flash it to openwrt version and It seems it is needed to upgrade by u-boot, somehow I cannot tell where is the u-boot console port.

The hardware have two DB9 com ports, which one is u-boot console or have to jump wire ?

Thanks!