Thank you for the reply  @Riley_S . 

Below are my answers.

What hardware version are your Decos? Updates have been pushed over the last few weeks to some older Deco models. 

I checked the deco router label, but i cant see any wording like "hardware version" and there is description given as US/1.6 after S/N. So I guess its v1 hardware version.

But to figure out what is happening:

Does the Main Unit also show Red? Yes Main unit and satellite units are showing Blinking Red. 

The Main unit showing red means an internet issue, while satellite nodes showing a red pulse indicates a disconnect from the main node. Try replacing the cable between your modem and main deco node and seeing if the Red LED still appears.

Do the LEDs on your Modem indicate that your ISP connection is still running? I'm using Motorola Modem MB8611 and is always having solid internet as per lights at front.

If this has been happening for this long and you have a v1 model - it is likely that a cable has gone bad in your network or you are having issues with the link between your Decos and your ISP.

If you have a more recent hardware version, make sure that it is using the most up to date version

Does the behavior exist when the nodes are communicating with a wireless backhaul? Issue is always between Main unit and Modem

Lastly, you may try changing your main node to one of the satellite nodes if you think that the problem is starting with the main. I will try swapping of units to see if it fixes.