@JohnnyF 

It may be that the distance the main Deco needs to cover is already enough. And also in that going directly to the main Deco is quick rather than connecting through the satellite. 

You could possibly try and find an area where the signal is very weak from the main Deco, place the second Deco there, once it connects move further away to see if your device then connects to the second node.

@Tony tried it, no luck.

Moved a secondary deco to the farest place from main deco, where the signal is weak.

Once the secondary deco was online, tried ti connect a device to it (device in close proximity). The signal from the main is so weak at this point that the device most of the time disconnects from main deco wifi, without noticing there is a deco near by! 
Something is very wrong here, I just bought this product and it doesn't work! 

@JohnnyF 

What is the device you are using to test?

@JohnnyF - frustrating in theory as it probably feels like the other nodes are redundant, is the signal/connection actually ok though? Or are you suffering as a result of devices only connecting to one Node? - edit just read the bit about being so far from the main node.

Have you checked that the Mesh feature is turned on for each node and device? This is the function that has them roaming between nodes