RPI GSM Add-on SIM800 AT commands are not working

Question

I am trying to send sms using RPI Add on v22 module and Raspberry-pi 3+.

I have mounted this module on top of Raspberry-pi.

When I use minicome using minicom -D /dev/ttyS0 -b 115200 -o

I get the terminal, but I can not type anything on that terminal. What's the problem there? My module is burnt?

My /boot/config.txt

dtoverlay=pi3-miniuart-bt
enable_uart=1
force_turbo=1

My /boot/cmdline.txt

dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait

My /lib/systemd/system/hciuart.service

After = dev-ttyS0.device

ExecStart = /usr/lib/hciattach /dev/ttyS0 bcm43xx 460800 noflow -

as given here.

My program is

import serial
import RPi.GPIO as GPIO
import os, time
GPIO.setmode(GPIO.BOARD)
port = serial.Serial("/dev/ttyS0", baudrate=9600, timeout=1)
port.flush()
port.write('AT'+'\r\n')
rcv = port.read(10)
print rcv
time.sleep(1)

In terminal I get nothing printed, at least AT command. If I change /dev/ttyS0 to /dev/ttyAMA1, AT command printed on terminal. But no ok coming from device.

Here they suggested to edit /etc/inittab, but I dont have any file there in my Pi.

I have separate 5V/2A power supply to the GSM module.

Added later by author: I have used minicom, sudo minicom -D /dev/ttyS0, but it has nothing

What am I missing here? I am new to these GSM modules. Any help would be greatly appreciated.