Turris shield blink twice

I bought second hand turris shield. When I power it it blink twice and then 1-2 second nothing and again blink twice. Do you know what it means?

Try to get into some rescue modes based on documentation, no luck. device behaves differently then in docs.
I have also not find any documentation how to prepare USB for reflash, did I miss it? Can you help there?

Thank you.