@nzrver 

It will work and you will have unified Deco mesh at your house. With your Smart TVs accessing Internet.

I checked X20-4G(LTE) specs and they show three Ethernet ports, they are WAN/LAN autosensing. There is no dedicated WAN port. If you plan to use X20-4G(LTE) with cellular network only, you may be able to utilize all three Ethernet ports for backhaul to Satellite Decos, without need for a switch to connect 3rd Satellite Deco. 

  @Alexandre. 

Thank you for answer. I appreciate that.

The LTE connection would be really a backup one. Instead there will be a cable connection directly to the router.

Is there any other option for a router from TP-Link family that will create a mesh with DECO APs? I don't necessarily need that X20-4G.

  @nzrver 

Deco will always run its own WiFi network. If you have found 4G (LTE) router that works for you, you can use Deco mesh in Access Point mode. Deco will provide WiFi coverage, router will provide Internet access. In that configuration, it is recommended to turn off router WiFi. See What’s the difference between Access Point mode and Router mode on the Deco?

  @Alexandre. 

I appreciate your answers.

I think I will go with different router (probably from ISP for time being, then I will upgrade to another one if this will be necessary). Instead of getting X20-4G, I will get another AP unit. 

Thank you Alexandre.