I have two ISP routers connected to TP link ER605 (both DHCP) and then Main Deco X20 connected to the LAN port of the ER 605. There are two more X20 connected  to the main X20  over Ethernet Back haul.

1. The Main X20 gets disconnected from the TPLink ER 605 and X20 starts showing a steady Red led (not flashing).

2. Rebooting the X20 gets the connection back again, but this again recurs with varying frequency. Couple of times the reconnection happens without a reboot but is unpredictable. I have waited for more than 10 mins a couple of times and then rebooted.

3. When I check on the ER 605 when the disconnection happens the X20 is no longer in the DHCP client list of ER 605. There is no other device connected to the ER 605 other than the X20.

4. I have checked that ER 605 and Deco X20 have the latest available firmware.

5. I have tried both the load balance without link failure and link failure options on the ER 605 - but the disconnection of the X20 happens irrespective of the ER 605 configuration.

When the disconnection happens both the ISPs are on and are giving steady 300mbps each when connected directly to the ISP routers. 

It is a month since I have the X20 mesh. The first two seeks there was no issue but these issues have started in the last few days.