If they all flash when plugged in I wouldn't imagine they're broken. Sounds to me like they are turned off in memory, change the LED mode and make sure brightness is on full.
In the programming software(can't remember the name of it) open the led menu and reset it to default, save and flash.
You mean O2D right? I did what you said, and still no underglow. I went ahead for more testing: I used a 3mm LED to touch the contact point of each underglow LED, and it lit up(as shown in the pic). Now I’m suspecting this is not a problem on software level....but let me know if this isn’t the right way to test.
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I recommend pulling out a multimeter, taking a look at the ws2812b data sheet, and just checking voltages between connected pins. The first LED might be borked, or you might have an issue with the microcontroller or its pins. Also check the USB port and see if there is cracked solder there. If you can't find an issue, maybe look into reflowing the microcontroller and the usb pins if you can do it safely. I'm sure that someone else will ahve better advice than me, but these are the guesses I can make from fixing my own board with basically the same architecture.