I’m not sure how to define ‘well connected’, so I’ll instead ask a question or three.

Are the radios (WiFi) turned on on both devices? 

How far is the mini-Sub from the Arc?

Any chance of wifi interference between the two, particularly in the 5Ghz range?

Hi, thank you for your prompt replay, but everything is connected under the same WiFi, as the Sonos app às recognize it as everything connected and define as one system (TV+Arc+MiniSub)

The mini sub is under the Arc divided only by a wooden shelf

I have try the ping of the mini sub and he is working well

I think is really something that I am not doing well or is something in the connection between the arc and the mini sub that it’s not working

I’d just make sure you have ‘bonded’ the Sub-Mini to your Sonos Arc using the Sonos App? 
Sonos App—> "Settings—> Sonos Arc room name—> Products”… it will be listed in that room if it is correctly setup and there will be the option to remove it aswell. (Don’t remove it, if it is setup correctly).

Alternately, I would recommend that you submit a system diagnostic, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Ok many thanks!