OpenWrt installation on Xiaomi R4AC

Hi there ...
I have a Xiaomi R4AC 100M Global and since it doesn't have an L2TP connection, I want to install OpenWRT on this device.
I tried everything and read every possible relative article and forums. But unfortunately, still I do have problems installing it.
Every time I try to install OpenWRT devices, I end up bricking and a slow blinking yellow light.

Please Help me ...

if everything else fails, use a serial connection, assuming there's one available.

Could you help me with it? I don't know how that works!

by bricking Xiaomi A4 I found out, that I got the 100M Version instead of the expectet Gigabit Version.

But with the Debrick Tool and method from here:
I got it back working.

But it was an old Firmware Version something like 2.18.28 on which the OpenWRTInvasion did not work. Upgrading to 2.18.58 made this method work too.
By having telnet I installed which worked.