@Kalinka 

Is the red/white flash happening on the Main Deco or the one connected Wirelessly?  

If its the wireless one, it could be that where it is positioned has interference in some way, something close by is broadcasting on a pattern and interfering.

Try running the network optimiser from the Deco App, this may change the channel and could irradicate it for you.   Else try repositioning the Deco (closer if possible) to the main deco and see if the red stops.

  @Philbert 

Nope, all of the Decos are flashing and optimization or moving doesnt work

  @Kalinka 

If its showing RED flashing on the main then its likely dropping the internet connection.     Do you have this dialed straight to the modem or via an ISP router?

The log you posted earlier is deffinately indicating that its dropping WAN connection, was this from the Modem or the Deco?  Has this issue only started to manifest or is this a new Deco install?

  @Philbert 

Its plugged directly into the modem and the errors are from the Deco system. I has happened before, but a few restarts usually solves the issue. It usually happens when a new device is connected or some change is made to the system settings etc. But now everything is as it was a week ago and it started losing connection for no apparent reason

  @Kalinka 

Ah ok perfect, is the Deco dialing the internet?  As in you have a Username / Password / PPPoE setup on the Deco?  or is this set for Auto?

If its Auto and the Deco is detecting the drop in the WAN connection, then guessing its something on the Modem.   What modem are you using.

If you run a constant ping to www.google.com  for example, does this drop out when the issue happens?  

  @Philbert 

Its Auto, with manual IPv4 and the modem is VSOL G/EPON ONU V2801D-1GT1
Ping is stable