We have a Mesh WiFi system for the DECO P9

  @Sevarch 

You can google for "Double NAT" and that will give you info on that topic, there should be articles at the technical level you are comfortable with.

For TP-Link Deco specifically, to eliminate "Double NAT" you will need to switch Deco Operating Mode to Access point. See Switching a Deco to Access Point (AP) Mode

In Access Point mode you may see faster WiFi speeds and better overall Deco performance. On the flip side, your Deco mesh will lose some of its features, you can read about it here: What’s the difference between Access Point mode and Router mode on the Deco?

If you need missing features, you can always switch Deco Operating Mode back to its default Router mode, but, then, you'll have "Double NAT" for XBOX back.