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

I’m a beginner with this…
Suddenly receiving error message “No heartbeat packs”
What does this mean and how do I correct it to make connection?