@Liwiu 

Check hardware version of your M4. If it is V4, see this: https://community.tp-link.com/en/home/forum/topic/581554

This is the feature description: How to select a preferred signal source on the Deco App

Also, in case you need it: How to find the hardware version on a TP-Link device

  @Alexandre. 
Thank you for your reply.

I checked the firmware version, it says 1.0.5 Build 20220512 Rel. 59723. It says it is up to date when I check for new updates.
Also, I should add that it is running in Acces Point mode if that's relevant.
The feature in the app that you linked is not to be found in my app, because of the Firmware version I assume.
Is there a newer version of the Firmware ?

  @Alexandre. 
Thanks for your help, just installed the new update and it worked.
I'm just a bit disappointed that it didn't help that much in terms of internet speed, at the main router I get 250mbps, and on the last one just 20-30. They are stacked over each other, just on different floors.

  @Liwiu 

Each WiFi mesh "hop" cuts speed by half. This is not just Deco, it is the limitation of WiFi mesh technology, for any brand. Which means, if your Main Deco delivers 250Mbps, Satellite connected to it will deliver up to 120Mbps, and Satellite connected to that Satellite will deliver at most 60Mbps - and this is in ideal conditions.

That's what you see.

That also explains why Satellite Deco prefers to connect to Main Deco. In many cases, bypassing an extra hop delivers faster speed, even if link to Main Deco is weaker than to the closer Satellite Deco.

The only thing left to do, and that will not change much, is to check that both Satellites getting Fair or Strong signal link:

If Satellite reports Weak signal, you need to move it closer to Deco it is connected to.

  @Alexandre. 
Thanks for the detailed explanation.
I might set up a deco on the 2nd floor as the main one because I have an internet port there, when I will use the Ethernet port from the deco placed at the 3d port (it won't be the main one anymore) what internet speed will I get on my pc, same as now or will it also get cut in half as the WiFi speed.

  @Liwiu 

I can't promise you exact speeds, but this would be my expectations with Main Deco on 2nd floor and Satellite Deco on 3rd floor.

Main Deco 2nd floor - same 250 Mbps as before

Satellite Deco 3rd floor, PC connected to it wirelessly - up to 125 Mbps

Satellite Deco 3rd floor, PC connected to it with Ethernet cable - for sure faster than wireless connection, and in theory up to 250 Mbps

I may have given the wrong answer. Please clarify:

You plan to have Main Deco on 2nd floor.

You will keep Satellite Deco on 3rd floor wired with Ethernet, using Ethernet backhaul.

Now, where will be your PC, to which Deco will it be connected and how: wirelessly or with Ethernet cable?