Won't boot up after a repair

Hello,

My sons have two *New* 3DSs, one red and one black. The red one was dropped and showed the behavior of a blue LED for a few seconds before turning off. I popped it open to investigate and used the iFixIt guide to remove the motherboard to check all its connections. Upon reassembly, the 3DS no longer powers up or charges. No LED lights whatsoever.

I thought I damaged the motherboard somehow, so I remove the motherboard from the working black 3DS and put it into the broken red 3DS. Same behavior. So I put everything back, and now BOTH 3DS won't turn on or charge! I've triple checked all the connections, and even reversed the tricky ribbon cable in the bottom corner of the motherboard, but I can't get any LED lights on either 3DS. What am I missing here? My current theory is that I had the cable backwards and blew a fuse on both. What do you think?

Thanks in advance

- Alex