@Markun 

First off let me say I do not have a Deco but have done a lot of research on it as I am considering using Mesh when I need to upgrade.

Now my question is can I connect the Xbox with a cable to the 2nd unit?  Yes. You can use the connection for either back haul or for connecting Ethernet devices.

If it's possible, does it give a better connection (more stable and significantly higher speed) That is something you will have to test. There are lot of variables involved. Is the WIFI speed between the Decos plus any overhead in the second Deco faster than the WIFI speed of the device you are connecting to and is the Ethernet port faster than the WIFI. You are only as fast as the slowest link. Since the Decos are designed to work between each other and use the latest tech, the speed should be faster between the Decos than it would be direct to the Xbox via WIFI. I would think it may be a bit more stable to use Ethernet connected Xbox to the second Deco as I think the WIFI between Decos would be more stable.

See https://www.tp-link.com/us/support/faq/2427/ for some more info.