One unit will not connect to other 5.

set up is as follows:

ISP modem > Main deco (1 of 3) > Switch > Deco (2 of 3), Deco (3 of 3), Deco (1 of 1 v2), Deco (1 of 1 v2), Deco X50 outdoor. 
 

all units working as expected bar 1 from the original 3, it will not connect by Backhaul, I have tried swapping it out with other units, same issue. Tried manual firmware update from v1.2.6 > v1.2.7 it was difficult to target this particular unit, as it would enter the reset cycle, yellow flashing, solid yellow, blue, then reconnect to the system over WiFi (never Ethernet) and then back to yellow after a short period connected as solid green (but only over WiFi) strong connection as I would ensure it's close during set up. 
 

To note, I can access it temporarily and can remove it, flash it or even rename it. Any devies  that connect to it via Ethernet port do not receive an IP address.

is it possible the unit is just a hardware fault?

I can't return it to the store as it was bought in another country. Please help. I would be happy to send the unit in for evaluation, or to allow an engineer remote into my network for a look at the config.

switch is netgear pro safe, loop detection is disabled. IPv4, ipv6 disabled (not supported by ISP). 1000mb FTH connection. As mentioned all worked perfectly up to about a week ago. Now I can't manage to get this one particular unit back online for longer than 2-3 mins with no Ethernet backhaul function nor rj45 function whatsoever from this unit. Constant solid yellow, short cycle flash yellow then back to solid. Only way to temporarily connect back to it is to reset it. But as mentioned this will drop off after about 3-4 mins and cause a yellow light loop.