@Dinreko 

If you configure Deco mesh to run in Access Point mode, it'll remove "double NAT" issue no matter how xbox is connected. See How to set up Deco to work in Access Point mode

  @Dinreko 

You're using your GPON as a router (HGU). you need to switch it to bridge (SFU) and let deco control the connection if you want to keep DECO in router mode.

Please check with your ISP to get access to the proper documentation to do this and to get the data needed to configure your master DECO.

  @Eric_Le_Grompf 

I found bridge mode in iptv settings. Is that correct?

  @Dinreko 

This is not in the DECO but in your ISP modem/router that you'll find this setting (ISP documentation or data is required). If you don't know how to do the job safely : please stick to the "AP mode". Anyway as it has been said : AP mode is the most simple way to get things done.

But a lot of people likes to keep "routing" on their DECO network because it is often "fancier" or for any other good reason i don't know.

  @Eric_Le_Grompf 

My network distributor said that deco can only work in router mode. So when I change in AP mode, nothing works

  @Dinreko 

Did you restart or renew all the IPs on all your devices ?

Switching to AP mode will remove the deco's private address range (and the DECO's router too). You'll be on your modem/router address range (as it features NAT there should also be a local DHCP server to provide you with the proper IP address. On the opposite if the DECO were needed to be routers there should have been no NAT in the GPON).

I had the IP renew issue on my network when i switched my X50 network to AP mode (it even went all red at one moment). You'll need to check status of all units with the DECO APP all along the process to get it right.