@David-TP Thanks for your reply. Here is my topology. It has been little bit better, when switched off "Smart DHCP" like others have been reported here as well. So Mesh is via Ethernet. There has not...

@TP-Link After updating M5 firmware to 1.6.0, main Deco looses LAN connection every day once, which didn't happen with earlier firmware 1.5.3. Would be nice to get updates without loosing stability....