@Flopsaut 

You should choose between M9 and X90, both are tri-band, X60 is not. That does matter. Out of these two, X90 will be most powerful plus it supports WiFi 6. It will also be much more expensive than M9. 

Your location is challenging for WiFi, so I would strongly recommend to buy mesh, whatever one you decide, from the place with easy return policy of at least 30 days. Also, you must have WiFi router now, so you could check if its WiFi signal reaches basement at all, and at what strength. If it is non-existent or extremely weak, when you stay in the basement right under WiFi router from 1st floor, chances Deco mesh will improve that are going to be slim. 

Have you checked if you have other options in networking connectivity between 1st floor and basement? If you have TV cable going straight from the basement to the first floor, you are in luck. If there are electric outlets sitting on same circuit breaker in both basement and 1st floor, that is an option to consider, too.

Thanks for the reply.

There is a signal with my current router. At least, I can get it with my cell phone.

But it is not very strong, so for the mesh solution, I would need a router that is as strong or stronger an emitter.

The hope is also that a mesh node would be a better receiver than my cell phone.

Regarding TV cable, I believe this might be a solution, too.

We get internet through cable connection and it seems the general hub for the whole building is in the basement, and actually in the part that I will rent.

How would this work ? Can one do the same as a powerline backhaul ? Does it require specific devices ?

@Flopsaut 

Ethernet through TV cable works on same idea as poweline adapters, but it is better. You could get gigabit or more over TV cable, and there is little noise in it usually, which means throughput will be better and consistent. 

This is the general idea:

It is MoCA ( Multimedia over Coax Alliance). You will need pair of MoCA adapters, and they will create an Ethernet link through TV cable. Attach Deco node to MoCA adapter on one side of that link, attach another Deco node to MoCA adapter on another side of that link, and you have Deco mesh talking over wired connection, between floors. Then, add wireless Deco units on each floor to have good coverage for each floor, how you planned.

These adapters are not cheap, but instead you could save on Deco (or other mesh) units. You might not even need tri-band mesh units in this setup, and that will be quite a saving. With just WiFi5 support, you might settle on Deco M4, and if you want future proof with WiFi6, you could get Deco X20 or X60 units for each floor. 

Research the MoCA, there are different standards. Anything offering over 1 gigabit is probably an overkill, but anything under gigabit should be dismissed.

Also, check your cable Internet modem/router: it could have support for MoCA already. If it does, that saves you one MoCA adapter - cable modem/router will play the role of another one. 

If this is the case, check documentation for cable modem/router, see what MoCA standard it supports and get additional adapter of same standard. Manufacturer web site of that cable modem/router might even suggest MoCA adapter brands and models.

Here is an example of MoCA configuration setting in my cable router Hitron CODA-4582:

Finally, it does not have to be just pair of MoCA adapters. If you have TV cable outlets in all right places, for example in rooms where you mostly use Internet, add MoCA adapters there, too, and attach Deco units to them. You'll have these Deco mesh units running with wired backhaul, which is always better than wireless backhaul.