@Rctabdo 

Welcome to the community.

Did you purchase the Deco in Brazil or from the USA?

As for the M5, have you tried to swap the location with the other M5, even X20?

And when it is disconnected, will its LED turn flashing red?

Could you please refer to this link to run a firmware recovery on Deco to see whether it made any difference?

How to use firmware recovery if the Deco become brick

Wait for your update.

best regards.

  @Rctabdo 

Thanks for the update.

I bought all the Decos in the US, but I live in Brazil.

Since both USA  and Brazil did not provide international warranty and later if you need a replacement, you might need to contact US support and provide a US continent address for the warranty service. Have you checked with the online retailer whether they are able to help you replace the Deco?

All others are working fine (three X20 and one M5). Only one M5 is not working. I've already switched with the other M5, with the other X20, with cable and without cable and nothing works.
I clicked on the link you sent, however I don't have a Windows PC, just Apple Mac.
Is it possible to do this on Mac?

I see there is no TFTP  tool for Mac yet and I am afraid the firmware recovery is not available on Apple Mac.

 

Best regards.