[Mutisk]

I know mine's problem was not getting access to an internet url. If you have a firewall or something blocking it will just shut down the wifi radio as soon it register the error.

My first clue was that it's IP address was still showing in my router, after that I could narrow it down to be assigned the same IP every time. Then while having a flood ping at the address I could get a few replies. And I got much more if the main internet was severed (my router times out in some hundred seconds)

Finally I redirected all the traffic from the kobo and analyzed it with wireshark to find out what it actually was doing.

It's tricky to debug, getting a telnet console helps a lot. But for that you need at least wifi for a short while. Should you get it, you can kill nickle to stop it from disconnecting (and everything else) until a reboot.

Edit: And yeah, hidden SSIDs are bad, ugly and wrong.

I can't possibly walk you trough all the possible steps to take and hope this post can at least serve as an inspiration to continue looking for clues. The more you find the more likely that we're able to help too.