@NLSOER 

It is possible your Main Deco M5 experiences hardware failure of some sort. To verify that, promote one of Satellite Deco to Main Deco. See Changing the Main Unit for your Mesh Network

  @Alexandre. 
 

Thanks for coming back so quickly.

I used one of the satellites as main and it had the same behaviour: WiFi active for two minutes and then flashing yellow, no more WiFi. Unable to update firmware for this one, two minutes very short to achieve that.

  @NLSOER 

Your test eliminated hardware issues with the single Deco unit. Unless you have two Deco units both having hardware issue simultaneously - which is quite unlikely. Also, blinking Yellow LED is not specifically documented for M5, only solid Yellow for that Deco model: What do the different LED colors meaning of Deco?

I can recommend two more things:

1. If both your Deco connect to the same extension power bar, plug them directly to wall electric sockets instead;

2. Configure Deco mesh to run in Access Point mode, 2 minutes should be enough to do that: Switching a Deco to Access Point (AP) Mode

With Deco mesh running in AP mode, connect all Deco units, Main and Satellites, directly to Ethernet ports of router. You should have four ports for that, that should be enough - Main Deco plus up to three Satellites. If you have more than three Satellites, keep the rest powered off. 

Also, don't use same extension power bar for all of them. See if multiple or all Deco switch to flashing Yellow (or to solid Red) and share results here.

If they all go flashing Yellow in that setup, the rest of troubleshooting is beyond my knowledge as advanced user of Deco mesh. You'll have to wait for TP-Link Support to check this topic and engage. They usually don't check that forum on weekends.