@Alexandre. Thank you very much for your prompt reply. The idea is to use DECO X50 or X55 or X60. Then set one DECO in router mode connected to one LAN port of the FTTH router and the other DECO in AP mode connected to the DECO router, ok??? Other issues: WIFI disabled on FTTH router, yes or not??? Which device assigns IP,s, the FTTH router or the DECO router??? Static IP can be assigned to the DECO router within the range of IP,s of the FTTH router??? Thanks a lot.

  @acerfly 

1. The idea is to use DECO X50 or X55 or X60.

Choose between X50 and X55. On top of more features in their firmware, they come with 3 Ethernet ports. X60 has 2 Ethernet ports. You may find extra Ethernet port handy if not now, then later. You may also find quite useful some features X50/55 have but X60 does not.

I recommend not to buy X60, if these are your three options.

2. Then set one DECO in router mode connected to one LAN port of the FTTH router and the other DECO in AP mode connected to the DECO router, ok?

TP-Link terminology is Main Deco and Satellite Deco. The one you connect to LAN port of the FTTH router will be Main Deco, the other will be Satellite Deco. If you add more Deco later, new Deco units will be Satellite Deco.

All Deco, Main and Satellite, will make one WiFi mesh.

You can connect Satellite Deco with Ethernet cable to Main Deco, just like on your diagram. Satellite Deco will use Ethernet link to communicate with Main Deco.

After initial configuration, Main Deco will run in Router mode. It will behave like WiFi router connected to the FTTH router, and you don't need another router on your network, it'll be a mess.

For that reason, configure Deco mesh to run in Access Point mode. Answers on remaining questions will be based on you following this recommendation.

3. WIFI disabled on FTTH router, yes or not?

Disabled. Deco mesh will provide WiFi coverage.

4. Which device assigns IP,s, the FTTH router or the DECO router?

The FTTH router. 

5. Static IP can be assigned to the DECO router within the range of IPs of the FTTH router?

Yes, you can do that on the FTTH router. The FTTH router will see Main Deco (and also Satellite Deco) as wired devices connected to it, with their IPs assigned by the FTTH router.

You can configure static IP for Main Deco (and for Satellite Deco if you want to) on the FTTH router.