Sounds like you might not have connected the cables quite right, probably one of the display cables. Open the console up again and try reseating them. The N2DS is notoriously picky about cable placement, so it might take a few tries. If that doesn't work, double check the rest of the cables too, and reseat any that look at all misaligned. I've heard of the ones near the ZL button causing a similar issue, so inspect those extra carefully
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