@juewi 

If you have Satellite Deco next to TV, it makes sense to connect them by Ethernet cable. Most TVs nowadays have Ethernet port.

Also, check the following FAQ document: Get to Know Fast Roaming / Seamless Roaming on a Deco System , scroll to "Q2: Why my clients roam to the Deco node farther away?"

  @Alexandre. Thanks for the answer. Yes, i had the idea to connect with ethernet-cable if all other WLAN-attends fail.

But i hoped it would exist an solution via deco-software for "forced" WLAN-connecting.

  @juewi 

You can't force connection with X20s and M4s. Here is the list of all Deco models that support this feature, and Deco mesh must consist only of these models to allow assigning clients to a specific Deco: https://community.tp-link.com/us/home/forum/topic/597476

From my experience with Sony TV connected wirelessly to nearby Deco unit, WiFi link is never ideal for video streaming. While watching 4K content, even slight and brief WiFi interference from somewhere forces streaming app on TV downgrade picture quality briefly, it is noticeable and gets annoying real fast. After wiring Deco and TV together by Ethernet cable, the problem went away.

  @Alexandre. 

Here is the answer from the support team:

Here you can check the steps.

https://www.tp-link.com/de/support/faq/3480/

The feature is not yet supported by all Deco models. The Deco M4 with the version V4 and the latest firmware supports it already, but the X20 not.
Please update the firmware and check if the feature is already added.

I have the X20 and the M4 version V2, so it doesn't work unfortunately.

But it´s no big problem, thanks anyway for your help