Not telemetry connection

I have SiK Telemetry Radio V3. when my computer is near to drone, i am able to connect pixhawk 6C with QGC with VTOL.

Entire antenna of the transmitter telemetry, (connected to drone) is outside the body.
And receiver antenna attached my computer is stick behind laptop with antenna facing up.

When VTOL starts going little further (around 15-20 m) or little high, the connection is disrupted and then there is no further connection between drone and QGC.

To connect the telemetry again, i have to land the drone, go near it.

What could be the cause and how to solve this to increase the range?