Can`t configure Mavlink mode 2 to the Telemetry kit

I have a 3DR telemetry kit, and a APM 2.6
I’m trying to send throught the telemetry link some headtracking data to move the gimbal.
I’m having poor performance in distances more than 15 m.
I also have problems for setting the mavlink 2 mode, that it seems to improve the performance in this cases. … tency_mode

When i try to set mavlink 2 mode in the configuration tool, or in the Ground station i get an error, and i could not set the link properly.

Any idea how to set Mavlink 2 mode?