Installing OpenWrt on Meraki MR42

Hello there,

Im trying to install OpenWRT on a Meraki MR42 via diagnostic mode as described here:

I've tried over and over to

Hold down reset at power on and keep holding, after around 10 seconds if the orange LED changes behaviour to begin flashing, proceed to release reset, then press reset two times. Ensure that the LED has turned blue.
but it always boots on solid orange, and after like 30secs it goes on rainbow for like 1sec, then blinkin green then dark then solid orange then rainbow.
I also dont see any activity on the TFTP server nor i could connect to (from .250) via ssh/telnet.

I've read that some of the MR42s just cant do diagnostic mode but i dont see/understand how to identify if this is one of them. It sucks because i dont have access to my UART converter for some time to do flash it the other way..

Any help is most appreciated,

I've realised that the sequence of led colors ive described above is basically the default startup one proceeded by the factory reset one. So basically holding the reset button at startup doesnt do trigger anything for me.