I didn’t buy the hacker pack but I used a USB to UART that I had for my previous openwrt router. If you check the board where the UART interface header is, you’ll see that it’s labeled “CN10”. Now, if you check the schematics from here: https://www.turris.cz/doc/en/howto/omnia_manuals
you can see what each pin is for. Starting from the side where CN10 is printed, they are: 3.3V, RX, TX, GND.
You don’t need 3.3V, just the other three, and don’t forget that the omnia’s RX goes to your adapters TX and vice versa. As for the settings, I used the default 115200 8N1 in minicom, but I had to turn off hardware flow control.