ZyWALL USG 20W - failed attempt to upload the software

You should be able to find the information in your manuals. Look for the "firmware recovery" chapter in the troubleshooting part of the Appendix in the manual, or use the CLI Reference Guide document to find out how to access a non-booting device in debug mode. You will need a serial cable (DB9 plug) and a terminal program to connect to your serial ports.

Connecting to the router when using a serial cable is not router specific. It is based on using any terminal-based software on your own computer and becoming familiar with the program and its settings. Some settings could be used as they are by default, sometimes you need to set specific settings to make the computer send the correct signals to the serial port to achieve the correct communication.

The default settings for the console port are as follows:

Speed: 115200 bps

Data bits: 8

Parity: None

Stop bit: 1

Flow control: Off

Not sure how it works on Linux, part of the output message saying /dev/ttyUSB0 makes me think that maybe it does not try to communicate on COM1 (default serial port)?

Unless there are some missing or unset settings to enable correct communication in your terminal software, I must defer to the Zyxel team members for more precise troubleshooting of your issue. Sorry for not being able to help any further.

I don't know enough about the procedure for specific models, I have only had previous experience with ZyWall 5 and USG 60, but that is now in the past and I am no longer responsible for following up on any Zyxel devices at my workplace. I can only suggest looking at other issue contents, unless someone from the Zyxel team replies here.

Maybe you want to repost the question, and maybe rephrase the issue?

Anyway, here is a link to a post from which you might possibly deduce some procedure information:

(Sorry that I won't be able to allocate more time to do any research into your issue.)