@David-TP 

Thank you for replying

-The wireless signal of the satellite Deco with the main Deco shows as weak with only one bar all the time. However, I tried to move the units to be near to the windows, still weak signal, the speed has improved but still not as expected.  

-Yes both 5GHz and 2.4GHz are used.
-I cannot test the Ethernet speed from the satellite Deco X50 as my laptop does not has Ethernet ports, but I imagine it will be not as expected also. 

Best Regards. 

  @itsAbdulaziz 

run the cable between floors, you can place the points closer to the windows to make it easier to lay the cable inside the house

this is the only correct and stable solution

  @Godlike66 

Thank you for your suggestion

I bought a mesh system to avoid running cables between floors :)

Based on the specification and reviews, I thought Deco X50 will be a good option for handling the network in the 2 floors, but I got a disappointed result, unfortunately. 

  @itsAbdulaziz 

Unfortunately, a similar result will happen with other similar devices.

mesh works more or less normally when there are no visible obstacles between the points and the number of points is small (2-3 maximum)

when there are obstacles and a large number of points, they need to be connected to each other with a cable