So, did you build this from scratch, starting with the Meggy Jr RGB PCB? If so, make sure that you’re using a preprogrammed “Arduino” ATmega328 (such as the preprogrammed ones from our shop), not a “bare” ATmega328 from the manufacturer.
In any case, that LED matrix has different internal wiring than the Betlux matrix– it’s sort of wired “inside out” compared to the other. If you look at the datasheets for the two matrices, you should be able to map how you’ll have to route the wiring to the matrix in order to get the display to work.
As for the cable, it’s very important to check the wiring order as I indicated, but you should also leave this for a secondary task *after* you get everything else up, running, and verified.
