UART communication between Rhino DC servo motor and RPi4 B

Question

I want to communicate with this Rhino DC servo motor with RPi4 B. I enable uart3 in RPi4 by modifying the /boot/config.txt file. In this file, I add dtoverlay=uart3 in the last. When I used ls /dev/ttyAMA* , I got /dev/ttyAMA0 /dev/ttyAMA1 , So I think uart3 is enable in RPi4.

For uart3, TXD is at GPIO04 and RXD is at GPIO05. So, I connected RXD of RPi with TX of motor and vice-versa. In this servo motor, to rotate we have to give commands like G200, G300,, etc. To get the current position, we have to send P. This link explains it in detail.

I used the below code to communicate with motor and RPi4:

import time
import serial

ser = serial.Serial(
        port='/dev/ttyAMA1', 
        baudrate = 9600,
        parity=serial.PARITY_NONE,
        stopbits=serial.STOPBITS_ONE,
        bytesize=serial.EIGHTBITS,
        timeout=2
)

#ser.write(str.encode(str('G600')+'\n'))
ser.write('G600'.encode('utf-8'))
val = ser.read('P')
print(val)
ser.close()

But there was no rotation of the motor (no error was there too). and I got value b' ' using print(val)

I want to know, how to run this servo motor with RPi4?