XPS 13 9315 LCD probem

First thing to do is check to make sure the cable from the display is firmly connected to the system board.  Service manual link below.

The cable isn't separately replaceable, so if it is damaged, it'll need to be replaced.  Depending on which screen resolution you have, that could be inexpensive (FHD) or very expensive (4K).

If you are seeing an image on the screen during the diagnostics -- including a backlight -- the cable is the likely cause.   If that's not the case, you'll need to evaluate whether to start swapping parts or to take the system to a repair shop so they can pinpoint the failure to the screen or the mainboard.  Sequentially replacing both parts will likely cost more than the system is worth.

I think its one this parts, cable or the connector between the motherboard and the cable.

But I saw it sold separately from lcd. And in my case it's UHD :/

If that's the UHD screen, bear in mind the assembly is fused together with strong adhesive. You may be able to find the cable -- but getting the screen apart successfully, replacing it and bonding the result back together without damage is a very difficult operation to perform successfully.