Ok. Little update: I was able to connect it again to particle cloud by using android app to give Duo WiFi credentials. And I observed quite weird behavior. A) It connects and signal works, but when it'd disconnected it still appears connected on the website.
B) I can't flash code. Website says that flashing was successful, but nothing happens to the Duo.
C) After some time it disconnects from the cloud (stops breathing cyan and starts breathing green)
I also reinstalled v0.3.1 using Duo CLI, but it didn't change anything. And earlier I confirmed that Duo has v0.3.1 and current bootloader (r5), so I think it's not the issue with downgrading.
But I think that reflashing bootloader might be at least worth trying. Any way to do that without RBLink?
And I think I saw somewhere that Duo has two more serial ports on pins (RX/TX and some other I don't remember). Can I access one of them by hooking usb cable data pins or by using Arduino UNO (as this is the only spare board I have right now)?
Anyone has idea what could have happend and how to fix it if none of my ideas above will help? Is it hardware issue, or just broken software? And if it's the hardware, then what can I do about it other than buying another Duo (if anything, of course)?