@David-TP 

Are all the Smart Home devices connected to the IOT network or Main Wi-Fi(2.4GHz)?... Connected to the main Wifi 2.4GHz, the IoT connection didnt work for me because the speed is very low, so some devices doesn't work fine when we're connecred to the IoT wifi.

-How many smart devices are connected to Deco Mesh?... More than 90

The latest firmware for Deco X55_V1/V1.6 is 1.5.5 now and it is also suggested to update to the latest firmware first.... Yes I know but this firmware was causing me a lot of problems with some of My devices, like My Smart TVs and specially the Eufy cameras, the AC controllers, so now finally I could connect all My devices and are working fine with the 1.5.1 Firmware, I did a looot of tedtd and things because I had a lot of problems with the wifi, but switch My Decos to A P mode and then downgraded the firmware was the solution.

But I am having now this problem if I have a power  or internet failure the Eufy cameras and the AC controllers can connect to the Deco Wifi after 10-14 hours, today I had an internet failure like 11am and the  cameras and AC controllers could connect again like 11:30 pm and now again all is working fine , but really this is very problemátic.

Please also disable "Fast roaming" and "beamforming" on the Deco APP.... yes I also did this because these options enabled we're causing me problems, so I disabled them and all works fine again 

I have these Decos

3 X55 (one is the main principal, all of them connected by ethernet)

2 X60 (one connected by ethernet and other by Wifi)

1 X50 Outdoor (connected by Wifi)

And a TP-Link TL-SG108 8 Port Gigabit Unmanaged Ethernet Network Switch to do the ethernet connection with the Decos.