@Gomi Could you describe your configuration more precisely?

I understand you have no Ethernet cables bewteen your Deco.

I understand that you have two "old wifi speakers", connected by Ethernet to one or to two Decos?

Each speaker connected to a single Deco?

Do those speakers communicate via wifi as well?

Could the Deco communicate via the speakers?

  @yves_b

You got it right.

Main deco has ethernet to the router.

Both satellites were connected to two separate speakers. They are Ikea Sonos wifi speakers. And no, they can't route traffic, but the Decos seem to think so.

  @Gomi Are you sure those Sonos do not transfer traffic?  I read some discussions on Internet about Sonos transfering traffic in such a setup.

Could you disable the wifi of one of those Sonos?

You wrote that you connected the Sonos by Ethernet "because the old speakers won't work on combined frequency networks".

You could perhaps activate the IOT network of the Deco, configure it on a single band (frequency), and connect the Sonos to it.

  @Gomi 

On Sonos system requirements page it states on a foot note that: “In WiFi mesh networks, Sonos products must be wired to the primary mesh node if using a wired setup”

https://community.tp-link.com/us/home/forum/topic/271216

  @yves_b 

No they don't. That's 100% of the point of a wireless mesh. It is very clearly rigged for both wired and wifeless backbone, and it works just fine if you don't use the nodes as downlink bridges for other wired devices.

New development though: I just need one of the speakers attached to one sattelite node and that node is not communicating correctly.

I've had this type of setup with one speaker attached to one node for a while, so this is new behavior. I thought it was because of the second speaker, but now I know something else has changed. Either the Deco firmware or maybe, but probably not, the speakers. They're old and I don't think they get updates at all.