@TP-Link As I said, the DECO M4s are configured as a Router, NOT as an AP.

The Freebox HDD is attached to the freebox and is/was accessible through upnp/DLNA from the freebox administered network (wired of fbx wifi).

In router mode, I had to shut down the Freebox wifi (it created some bad routing behavior) :

(A) is it absolutely necessary to shut down the box wifi with DECO in router mode ?

In router mode, the freebox TV was correctly routed to my player (I used RJ45 between the last DECO and the player).

I have not found a way to propagate the DFLNA visibility to the second network created by the mesh Wifi, but if you know how to configure the DECOs for this let me know.

This is the purpose of my question. 

I have changed the DECO mode to AP and indeed the HDD is now visible.

(B) Can I access DLNA with DECO in router mode ?

and :

(C) : when a device is attzched to a secondary DECO (Router mode) with RJ45, does it see the primary network or the secondary ?

Thx if you can answer the three questions above.

The technical document is probably okay ofr beginners but it is lacking technical details for the router features (which seem to be also a bit limited).

Maybe the App is limiting access the the advanced features, for 15+ years, I have handled routers configuration through web interfaces, several of them were TP-Link routers.
Adding an http settings server on the DECO would be welcome for advanced users.

Note that I found some bugs even after upgrading to the latest firmware (like LED management).

BR