@BiancaAlfario 

Deco mesh will not connect with non-Deco equipment, but I can offer you simple solution for what you want.

You will need to get Deco X50-PoE. This exact Deco model: https://www.tp-link.com/en/home-networking/deco/deco-x50-poe/

Deco X50-PoE is compatible with Deco X50 you have, will join your Deco mesh, is WiFi6, and you can still use homeshield. By itself, Deco X50-PoE is wall and ceiling mountable, so in some places that may be enough.

If you specifically need "plug-in style" device on electric socket, you can achieve that by buying third party wall mount holder for X50-PoE. In Amazon in your region search for:

SLEEMAKE Wi-Fi Wall Mount for TP-Link Deco X50-PoE Whole Home Mesh WiFi 6 System, Space-Saving Outlet Mount Bracket Holder 

If these are not available in Amazon of your region, here are the pictures of it so that you can search for different brand of similar product, understanding how it looks:

  @Alexandre. This is awesome and looks great. However the price of single x50 Poe equals the price of  2x x50 in my region. I was googling to find if there are a similar holders for the regular x50 but couldn't find anything what would hod on electric socket. These devices caa be screwed to the wall only I guess. Did you come across a similar holders for a bigger device like x50 ? 

  @BiancaAlfario 

I would not recommend to try that for bigger devices such as X50. Even if you could find similar type holder for it, the total set will be heavy, bulky and easy to trip over. You may be able to find wall mount holder for X50 that you will have to attach to the wall, but it is the same problem. 

For what you asked, Deco X50-POE is the only solution. Because you have quite specific requirements, it should be expected that it is going to cost more. 

Check Deco X50-POE price for a few days. I am in Canada, in my region Amazon offers it right now on 16% off sale for $125 CAD from regular price of $150 CAD. You might catch sale in your region, but if not, it is still the only solution which fits all your requirements.

The only other option is Deco M5 or M9 Plus with their third-party wall mount holders. These Deco models are WiFi5, and that is what I bought 3 years ago as I needed to wall mount one Deco unit in a narrow corridor. When time comes to upgrade them, I'll go with X50-POE in setup I shared with you.

Deco M5 and M9 Plus are WiFi5, you are asking for WiFi6 (which is correct for what you already have), that's why I didn't recommend them to you.

  @BiancaAlfario found it in Itally with 35% discount :) thanks 

https://www.amazon.it/dp/B0BSFFJN93?tag=haggle-web-it-21&linkCode=ogi&th=1&psc=1