@David-TP 

Thank you David!

The 3rd party device is a Sony Bravia KDL-40EX1 digital TV consisting of a TV screen and media box connected through a wireless connection.

I was wrong about the bandwidth, according to the manual the TV uses OFDM wireless transmission on the 5.15 to 5.25 GHz range.

I have now changed the Deco to provide 2.4 GHz WLAN only, but the problem is still there.

So I should rephrase the question: Is it possible to disable the 5 GHz band in the Backhaul?

Unfortunately it is physically impossible to rearrange mesh routers or to connect them with Ethernet cables.

Any other ideas?

Many thanks

Jorgen