High latency protocol configuration in Pixhawk 2.4.8

We are trying to set up high latency link on telemetry 1/2 port of Pixhawk 2.4.8 flight controller.
In case of QGC software, we have configured communication link to high latency and at 9600 baud rate. We have connected Pixhawk Telemetry port 1 to PC using RS-232 COM port. When we connect the link after few seconds QGroundControl says that it did not get a response to the command MAV_CMD_CONTROL_HIGH_LATENCY. Kindly help to solve this issue.

In case of Mission Planner, we are unable to find out how to set telemetry port protocol to option MAVLink High Latency. We could not find Serialn_Protocol in Full parameters list or full parameters tree under CONFIG tab. Kindly help to solve this.