Checking 4g balance via USSD on TL-MR6400

I have set up my TL-MR6400 v5 router successfully. One thing I am stuck at from the LTE QMI guide is this part:

To check your balance or send any other AT commands, you need to have usb serial device like: /dev/ttyUSB0

Following the steps outlined after that statement yielded no results so my question is: how do I check if my router model has a serial device or some other form of hardware connectivity medium ?

ls /dev on the CLI.

I've found ttyUSB0, ttyUSB1, ttyUSB2, ttyUSB3
Yet running cat /dev/ttyUSB0 in a new terminal window then running the command: echo -ne 'AT+CUSD=1,"*222#",15\r\n' > /dev/ttyUSB0 doesn't get me any output in the new window.
My guess is that the device ttyUSB0 is either not the serial usb device to send AT commands from or it IS the correct device but was not configured properly.

EDIT: Switching to ttyUSB2 gives errors, better than nothing:

cat /dev/ttyUSB2
+CSQ: 27,99