Hi @Alexandre.,

Thank you for your reply.
I know that-theoretically-different Deco will work properly together.
But I found advice to buy the same Deco as exisiting one, instead of buing higher model:
https://community.tp-link.com/en/home/forum/topic/607254?replyId=1202430 
  

  @skansen 

Deco M4 supports up to WiFi5. Deco X50 supports newer WiFi6 in addition to WiFi5. 

In situation like yours, when you want to expand Deco M4 mesh, my personal preference would be one of the following two:

1. Add another M4 or M5, so that all Deco mesh units are consistent in supporting WiFi5;

or

2. Replace all M4 with X50, so that all Deco mesh units will be consistent in supporting WiFi6.

There are no Deco mesh features I can think of you will lose by adding X50s to M4s.

The only question is how device that is connected on WiFi6 to X50 will roam to WiFi5 on M4. The roaming logic is device specific, for example device may stick with WiFi6 until it almost loses signal instead of roaming to M4 sooner. You would not know until you have your mixed Deco mesh and try with different mobile devices you have. This may be non-issue, but who knows. That's the reason to keep WiFi mesh consistent.

  @Alexandre. : thank you for sharing your thoughts. 

Does anybody use WiFi5 deco units tohether with WiFi6 units and can share some feedback ?