Ardupilot doesn't understand heading message

Hallo, Ardupilot comunity!
I’m trying to use GPS for Yaw feature in my autonomous boat.
There are no any issues with two Ublox F9P resievers, connected to Uart 3 and 4.
Autopilot firmware version - 4.2.3
But in my setup better to use internal moving baseline processing to calculate heading using f9ps itself.
So, I connected my recievers via cross cable, set them up sucessfuly and then connected to Uart3 of my Pixhawk, then followed instructions on GPS_For_yaw, provided on Ardupilot docs cite.
On the main screen I can see rtk Fixed, but no heading from GPS. Mavlink monitor also shows 0.

GPS Auto Config is 0
GPS Type - 5
EK3_SRC1_YAW - 2

Looks like Ublox can only send Heading information by sending RELPOSNED mesage, but ardupilot cannot recognize it.

Help me please, i have no idea what to do with my system. May be I forgot to set some parameters, but i cannot undestand which exactly.
Thank you!