@Ed72 

I can confirm that Satellite M9 Plus with wireless backhaul provides faster WiFi than Satellite M5. In my tests the best I can get from Satellite M5 was 225Mbps, for Satellite M9 Plus the peak was 490 Mbps, in the same location of my house and connected to same Main Deco.

These are almost ideal test conditions and best results, the usual speed from Satellite M9 in my house is around 350Mbps. My Internet connection is gigabit.

Tri-band M9 uses 5GHz for third, dedicated backhaul, band. If 5GHz connection between two M9s is weak, if there are obstacles or electronic noise, Satellite M9 might perform no better than Satellite M5, and might even do worse.

The first thing to check is quality of signal reported by your 2nd floor M9 when it is active (green LED). In Deco app, select that M9 and see what it shows, you should expect to see this:

This is my Satellite M9, shows all three bands and very good signal quality to "Living Room" Deco M9 it is connected to. If you see less than three bands, just 2.4GHz, weak signal with orange or yellow colors - that means your 2nd floor M9 is not optimally located. You should move it closer to the master bedroom M9.

Also, this will show you which Deco your 2nd floor M9 is connected to. Perhaps, it is trying to connect to Main Deco, that would be known issue TP-Link is working to address with firmware version 1.6.

@Alexandre. 
 

Thanks for that!

I did check that a couple of times and the 2nd floor M9 keeps being connected (when it works) to the master bedroom one (which is wired). 
 

The thing I do see is that the signal strength drops: after just having installed it, it shows all blue bars (so full strength), but this drops to two bars and sometimes even one bar. Without an excessive load on either two of the M9s or moving them of course. 
 

The M9s I now moved upstairs have always worked perfectly when they were located downstairs. So I also know they should work fine, but right now I do not feel like that at all and am even thinking about returning the X90s and reverting back to my M9/5 setup. Which is of course not what I want, because that way I won't reach the high wireless speeds. 

this was my situation a minute ago (after resetting it and adding it to my network again). 
 

But now it has lost connection again and is in its annoying red blinking state again. 

@Ed72 

I wonder if you have some sort of wireless noise or interference in your house. 

It makes sense to return to original working configuration. Still, I would suggest you try, as a test, moving 2nd floor M9 closer to the master bedroom M9. It is somewhat common misconception that one must put Satellite Deco unit with WiFi backhaul same place where wireless devices are that will connect to it. In fact, the better place for Satellite Deco using WiFi backhaul is somewhere in the middle between wireless devices and Deco it is connected to.

Only when Satellite Deco is wired, you should place it as close to wireless devices as possible.

In your case, place Satellite M9 servicing 2nd floor bedroom in the middle between master bedroom M9 and 2nd floor bedroom. It does not have to be exact "in the middle" - for example, you can place that Satellite M9 on the first floor but under the 2nd floor bedroom.

Run this as a test, all you need is electric outlet(s), try different ones in a few places, see if you could find a better location for that Satellite M9.

By the way, that does not apply just to M9. It is applicable to any Deco model and to any WiFi mesh in general: Satellite units that are wired should go as close to wireless devices as possible, Satellite units with wireless backhaul must be placed in the middle between mesh unit they connect to and wireless devices.

Finally, if you really have time on your hands, you can run one more test after you rollback to original configuration but before you returned X90s. Replace 2nd floor M9 and master bedroom M9 with X90s you have available now, and see if X90 to X90 connectivity has same problem. 

In the future, you may decide to upgrade your house WiFi mesh to WiFi6, it would make sense to know in advance if X90s do or do not suffer like M9s in your house.

@Alexandre.

Thanks for your tips Alexandre! I will try the roll back during the weekend (easier than during the work week when we have two people working from home in constant need of a signal). 
 

I will for sure test the X90 upstairs and was even thinking about using a M9 as my main Deco and have the two X90's connected to it wired. I know TP advises to make the X90 the main Deco, but in my case this just works as a router for the rest. I do not actually use its wireless signal (since it is tucked away in the meter box) and only use the WiFi signals of the living room and master bed room as main WiFi. 
 

So I don't know if that is a possibility for now, but I thought it would be worth a try. 

@TP-Link 

Thanks for your reply. 
 

The network structure is indeed as pictured in the diagram you made. 
 

As for swapping the M9s: I will give this a try during the weekend as well. 
 

Could you also confirm whether the idea to make an M9 the main Deco and the two X90s the wired ones in the ground floor living room and the first floor bed room would or could work?
 

For now I have changed to 2nd floor M9 back to an M5 and this one does, as always was the case, keep its connection.