• What is the Wi-Fi card you’re using? Some cards using a Realtek chipset have bad performance.
• Check and see if the antenna is being obstructed by the case on S920, or if there’s anything metal nearby that could be causing interference.

As to how you can go about debugging it, that part is hard. I’ve suffered from frequent disconnections on my OpenWRT install on a router, and I had to trawl through the system logs (it’s there on LuCI) to see what was triggering the disconnection when using the AP. After you find the line you think could be it, Google for it to see if anyone else has had similar issues. Ask on the OpenWRT forums since people on there might have come across it and have a solution.

You should also check to see if a reconfig of a very basic AP triggers the issue.