@terziyski thank you for your answer.

Yes, I tried to enter the DNS address in both the Internet settings and the LAN settings.

Maybe, I have found the problem.
I had enabled "Access Control" - "Whitelist".

When turned it off the Internet appeared on Box.
But after reloading Box, the connection was lost again.

I reset the router to factory settings again.

The connection between the router and the TV box does not happen immediately at startup, I need to wait 1-2 minutes.
But another router connects at the startup stage, I immediately see the IP address on the Box.

Oh, I also activated IPv6, just turned ON this option.

Will test more.

The router is not working properly anyway.
The wired connection disappears after a while (possibly when switching channels), and the IP address disappears in the box.

There are no such problems with WiFi.

Approximately every half hour or hour in the TV box, the wire connection disappears.

Here is the router log file.
I try to turn different options on and off.

Tv box Lan: A8-20-0A-AD-38-00
Tv box WiFi: CC-8C-DA-8A-7B-90

Hello @Kevin_Z .
If you need it, you can remotely connect to my router.

Yes, I reserved the IP address 192.168.0.12, because it has a Samba server.
Now I have removed all reserved addresses. I'll check it out. But the first connection didn't work better, the router and box didn't connect quickly.

Thanks you.

ps. Nothing has changed, Box has lost its IP address again.

Now I do not use Samba server and Box has default parameters (get ip from DHCP server).

I found a solution to my problem.

My TV box cannot establish a stable cable connection if the cable has problems.

But, this cable works great with Desktop and TV.
But, if I connect the cable in reverse order (swap the ends), then the TV box works fine.

So if your Ethernet does not work or it does not work stably:
- connect the cable in reverse order (swap the ends).
- replace the cable with a new one.
- double-check your wall sockets.

Thanks to TP-Link Support for your patience and help.