@Nrgzer 

Hey

This is likely a signal issue

2.4ghz while slower and more congested does have a slight advantage over 5ghz in terms of penetration.  The longer wavelength allows for better penetration of walls, obstacles etc but at the expense of speed.

If its connected at 2.4ghz but not at 5ghz, then its likely that the 5ghz is too weak to establish a solid connection.  Try moving it closer, or run network optimiser and see if a channel change helps!

Hi  @Philbert 

I've run setup next to the main unit a number of times and the result is always the same. Something is wrong with this specific unit in my opinion, the other two work well.

  @Nrgzer 

If its doing it at short range then yes its possibly faulty.  

Have you tried resetting that specific Node?

  @Philbert 

Yes, I've done it a number of times but the problem persists. 

Hi  @Philbert 

I'm in Brazil and the item has been purchased here.
Can you provide the details of where the Deco M5 units can be sent for test and replacement?

Regards,

  @Nrgzer 

Hey

I am not actually a TP Link Employee, just another forum user like yourself.   You would be better contacting support directly and asking them for RMA process. 

Details below (US site)

https://www.tp-link.com/us/support/replacement-warranty/

  @Philbert , many thanks for taking the time and the replay.

All the best.