Bricked TP-Link WR802N

I installed OpenWRT to my Access Point but then I wanted to switch to the old firmware. I flashed the old firmware and now my AP is bricked. I connected it to my PC over ethernet but connection goes up for about a second and goes down very quickly. Light turn on for about a second and turn off again in sync with the connection. I looked at the Package dump and there's nothing but ARP requests by my PC when I use static IP address, when I use DHCP there's only DHCP request. I tried changing power adapters, cables and waiting for a while but nothing helped. I tried TFTP firmware update but it doesn't work either.

in that case, you probably need to hook it up to a TTL USB serial adapter, to see what's going on.

How can I do that?

Well, you need to start by opening it up, and see if there are any solder points or pins for a serial connection.