@Yorocoboy I 've tested more this issue with original passive poe injector but using smaller cable (fixed 3m) and its working fine as you mentioned in your post.
Then, I "played" with AP's wifi settings and i noticed that the problem is only when i watch heavy videos from 5 GHz antenna network. So I connected again my first 20m FTP cable, and when i reduced 5 GHz Transmit Power setting to "Low" & Wireless Mode is "802.11a/n mixed", then reboots reduced, even when i watched any heavy videos (fb, youtube, video apps) from that (5 GHz) network.
So i confirm the issue has to do with power dropout when transmit power of 5 GHz network is middle and above.
I believe, problem with 2.4 Ghz network doesn't exist, because transmit power from that network doesn't raised up to the limits of power adapter.
We've been beta testers as company let us be. Are we going to see a fix for that issue we confirmed in the near feature with a FW update, or even tell us to use a stronger power adapter with specific voltage/amperage? We can wait for that, but we hope no so much...