@soxac 

I would suggest this approach: build WiFi mesh with what you already have, and when you become familiar with Deco mesh and its capabilities- make changes and/or buy new hardware for your needs.

So, starting with:

1. You do have two M9 Plus. I am familiar with those, they are part of my Deco mesh;

2. You do have "modem/router" already, to which your AP are connected: what is this modem/router brand and model name? Is it VR2100?

You ask if VR2100 can run same WiFI mesh as Deco, the answer is no. Deco mesh will run its own WiFi network. You can run Deco mesh in Access Point mode with extra DSL router, with that setup you'll turn off router's WiFi and can use all other router features. Deco mesh will provide WiFi coverage.

Alternatively, you could get Deco X50-dsl or X20-dsl and make it your Main Deco, running Deco mesh in Router mode. The benefit is you'll have everything under WiFi mesh and no extra device, the drawback is you might find Deco router features quite limited and troubleshooting DSL issues quite challenging.

So, if you do have DLS modem/router already, I would recommend Deco mesh in Access Point mode and if you are on board with that - will explain how to set everything properly.

  @Alexandre. 

Hi Alexandre,
thanks a lot for describing the options. I currently have a TP-Link TD-W9960. You are right that it is more reasonable to start with what I have and only then think about possible additions. You describe 2 options, however I see one more...
What to use the existing Modem/Router almost only as a "modem" and use one of the M9plus as a Router? This should then have the advantages of one Mesh network, is not it true? Then it would also be possible to exchange the modem for another DSL at any time, or for another connectivity source. But maybe that's a bad line of reasoning based on the assumption that a single Mesh network is significantly better than separate APs.
Do you have any experience using the M9plus for Zigbee? Doesn't the whole house network need to be under that M9plus router for it to work properly?
Thanks a lot

  @soxac 

I have no experience with zigbee, can't help you with that.

Does not matter how you run Deco mesh: in AP or in Router mode, it will provide WiFi mesh for your house.

If you want to run Deco mesh in its default Router mode, this will be your network diagram:

This should be working just fine, as long as switch is compatible with Deco Ethernet backhaul. I forgot to mention this before: in both Deco AP and Router mode, to use Deco units connected through the switch, that switch must support Deco Ethernet backhaul.

More about this topic here: General questions about Ethernet Backhaul feature on your Deco

  @soxac 

With DSL, I suggest having a separate modem, and connecting the Deco mesh to the modem.  Choose a modem with minimal capabilities, compatible with your ISP.
When you go to fiber, you can probably directly connect the Deco mesh via an Ethernet cable to your ISP.

Disable the wifi on your modem.

If you can disable the router mode (dhcp) of the modem, then configure the Deco in Router mode, otherwise run your Deco in AP mode.

When the Deco runs in Router mode, be sure that the main Deco is connected directly to the modem, and everything else is connected to the main Deco, not to the modem.  In your case, connect the switch to the Deco, not to the modem.

If possible, it is very beneficial to interconnect the Deco by cable rather than by wifi.  It reduces latency and improves bandwidth.  
The benefit will depend on your usage and the bandwidth contracted with your ISP.