How you do connect to jetson using serial cable? What program do you use? What are the settings?
Are there any other way how to troubleshoot the device? In the situation when it seems the device is not booting (no ethernet connection) neither serial connection is working?
It’s the DB-9 connector. Use a “null modem” cable to your desktop, or to a serial UART adapter USB cable. Setting is 115200 speed, 8n1. Sometimes there is a setting for flow control which will work on no flow control or with XON/XOFF (I use the latter).
Any serial console program works. I use mostly gtkterm, others are minicom or PuTTY (this latter on windows). Under linux you will need permission for serial port access, so it could mean just using root or sudo on your gtkterm or other program, or perhaps a user with “dialout” group (whatever group covers serial ports).
Debugging would depend on results of serial console connect.