SamyGO

I don't think you could play that MKV's but AVIs...
But installing latest firmware is really big problem on LExxB550's because of lacking ExLink and protected firmware for hacks...

I inspect your FW and find some LX51 files. Don't know this format I can say that your firmware is really different than ours.
Nothing to do.

If you have that firmware, it's possible. (With 2008.0 version.)

There is also another way, I don't know if this hack work on you
You can check it via : http://wiki.samygo.tv/index.php?title=E ... es_Devices

And you can test T-CHL5DEUC firmware via booting that firmware via USB Flash.
There is also an article at wiki describe this but I have no experience on it: http://wiki.samygo.tv/index.php?title=B ... ing_u-boot