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Pixhawk connect MP by 4G telemetry "No Heartbeat Packets Received"

I wanna make a 4G telemetry(raspberry pi & 4G dongle)
and follow the step of this video:

However, I got the message “No Heartbeat Packets Received”
I don’t know what the problem is
Can anyone help?

following videos are how I connect and RPI can successfully connect to pixhawk

no heartbeat packets received

raspberry pi to pixhawk

Servers by jDrones