@FQuesnel 

The solution would be to get Ethernet cable from the basement to the second floor and connect satellite Deco to it. That'll make it as fast as Main Deco.

If you have TV cable that goes from the basement to the 2nd floor, you can use that to pass Ethernet traffic, with MoCA adapters.

If you can get cable between first and second floors, and connect both Satellite Deco with it, your second floor will become as fast as first. Not 500mbps, but definite improvement.

Otherwise, you will have to replace your WiFi mesh hardware. For speeds you want you should be looking for Deco models that support WiFi 6 and are tri-band. That would be more expensive than M4R. Also, results are not guaranteed. Everyone's house and what it is built of are different. 

So, I would suggest you check TV cables at your house or how to get Ethernet cable between floors.

  @Alexandre. 

Thanks for your reply. I guess that makes sense, I don't have tv cable in the house (also works wifi) but bringing a Cat5e from the modem to the second floor may not be too difficult as there is an abs pipe in my walls holding a couple of electrical wires going from the attic straight to the basement.   I had totally forgotten this. Cable would need to be about 60' long, probably short enough to preserve much of the speed.  

Many thanks!