Synology router RT1900ac wireless disconnects frequently solved

I had suddenly a weird problem with my wireless on my synology router RT1900ac.

The wireless showed a full signal on my devices, but when I connected with any device it disconnected within seconds. E.g. on my phone it showed: disabled.

Never had any problem before and then suddenly today this occurs. Weird.

Luckely, after a few hours searching, I found a solution: put the transmit power to LOW of both 2.4 and 5 GHZ. Once I did that, the connection remained stable.

Question remains: why does this suddenly happen? Nothing changed, no firmware upgrade, nothing at all...