@CameraF3 

You have to bear in mind that a range extender is NOT a mesh device, its literally a repeater..   It takes in what is coming from the WiFi and repeats it out again.   For that reason its an almost passive device and wont appear in the Deco App as a node.    In short it will repeat and extend a mesh, but wont become part of the mesh as its not a node.. just a repeater

You would be better with a cheaper Deco node, something like the E4 or S4 as it will have the full deco node support and not just 'repeat the signal'

  @Philbert 

no it's a TP Link , One Mesh extender. 
Thats really my whole point, the sales implication is that it works with the TP Link mesh system.

  @CameraF3 

Hello,

"Déco mesh" and "one mesh" are based on two différent technologies : first is real full mesh , the second is an extension of the usual wifi architecture mode with a unique central Access point for which the extenders are made for. Extenders usually cannot Relay like Real mesh due to the penalty of heavy losses. Tp-link seems to know a workaround with one mesh. The two tech are not compatible. The first tech uses multiple radios like cellphones stations to get coverage , the second one uses muscles to irradiates all around.

Anyway, there's a publication about this incompatibility on tp-link blog.

Regards

  @CameraF3 

Hey

@Eric_Le_Grompf has summed this up perfectly, yes they are both 'Mesh' in a matter of speaking but very different under the hood.. deffiantely not the same thing.   

If you are able to return the extender you should look at a E4 or S4 Deco in a single unit, these are the cheaper end AC grade devices but should be sufficent for what you require.   All Deco nodes work together without issue.