Are you using the “add new Sonos” option in your App that is connected to your existing system?

If all of your other SONOS speakers are working, Factory Reset (only) the BOOST, wire it to your router, and add BOOST to your system as as if it is a new unit. In general Factory Reset does not solve fundamental issues. In this particular case I don’t know exactly what and when something has been Factory Reset. Boost might have been setup as a different household.

Of course, we cannot rule out hardware failure at this point.

To add Boost, I go to Settings/Systems/Add a Product… but it does not find my Boost despite the fact that I resetted it. 

Despite doing this, it does not “see” the Boost. 

Where are you expecting to ‘see’ it?  It won’t appear in the System screen, which lists all the speakers.  Does it appear in ‘About My System’?

Ok. Before, my Boost was connected via a hub, and it was working well. 

I just tried to connect the Boost directly to my internet box… and it works… So the problem seems that I cannot use my hub anymore… which is an issue for me… I only have 2 ports on my internet box (one for my TV box and one for my hub)…. Strange that it no longer works through the hub…

Now, supposing I manage to connect my Boost, do I have to instruct each speaker to use the Boost or is this done automatically? Can I see this somewhere in the Settings?

This is done automatically. Speakers are connected via the Boost when their status shows WM:0 - surrounds will show WM:2.

What is a “hub”? A simple switch like a Netgear GS105e should suffice.

If there is a wired  Sonos device, and the system is therefore in SonosNet mode, the surrounds would show WM=0.

Like @106rallye , I am not clear what you mean by ‘internet box’ and ‘hub’.

By hub, I mean Netgear switch. If I connect the Boost on it, it does not work anymore

Internet box: the box provided by my internet supplier where I can connect devices (only 2 ports)

Hub: a Netgear switch with 8 ports. When I connect the Boost on one of these ports, then, it does not work… While it was working well before. I tried several ports without success. The ports work well with other devices. 

Which model of Netgear switch?

GS308v3

Okay, a dumb switch. Not on the ‘incompatible’ list. No obvious reason why that shouldn’t work with Sonos just fine. 

Offhand I don’t know why BOOST would not work when connected to your switch. What is a “TV box”?

One thing to try is to plug BOOST into the router, then connect your “TV Box” into BOOST. Yes, I know, some will chide me and remark that BOOST’s network speed is only 100Mbps, but this is probably sufficient for your application. Again, I don’t know why you would need to do this, but if it works OK, declare victory and move on.