@Adrz 

In Deco app, open More/Advanced and check if you have Smart DHCP setting there. If you do, is it set to Enabled or Disabled? 

  @Alexandre. Thanks for reply.

I don't have this option "Smart DHCP", maybe it doesn't exist on Deco M4...

Hello  @David-TP , thanks for reply

 Did the main Deco M4 turn red after restarting?

 Yes, but after few minutes it turns white. The second Deco M4 has red led until I restart the router.

Would temporarily unplug/plug the Ethernet cable between ISP router and main Deco help, instead of rebooting the ISP router?

I tried but not works.

By the way, is the ISP router the only device that connected to the main Deco M4 via an Ethernet cable?

No, there are other siwtches and devices.

Well, to sum up, here are some details that I have seen when performing these tests:

Restart the main Deco or remove power from the main deco and reconnect or disconnect the internet cable and reconnect

- No WiFi device can connect. The main Deco starts yellow light, then red and then white.

- The second Deco M4 has red light and any Wi-Fi device can be connected until I restart the ISP router.
- If I connect a device by ethernet cable it cannot obtain IP from the router.

- If I disconnect the ethernet cable from de main Deco, all ethernet devices works fine (before and after the restart).

This behaviour seems very very weird, looks like a compatibility issue between my ISP router and Deco M4?

Hello  @David-TP , thank you for help and reply,
 

is the satellite Deco M4 wirelessly connected to the main Deco M4?

Yes, I live in a two-story house and the main Deco connects with the secondary (satellite) Deco wirelessly.

And are you able to constantly reproduce this issue?
Yes only when I reboot from APP/Web, disconnect/connect the power and disconnect/connect the ethernet cable.

Thank you in advance