TP-Link 450Mbps Wireless N Router
Model: TL-WR940N (EU) Ver: 6.0

TP-Link AC1200 Whole Home Mesh Wi-Fi System (3-Pack)
Model: Deco W4R Ver: 2.0

SSID: TP-Link A2C6

I recently bought a TP-Link AC1200 Whole Home Mesh Wi-Fi System (3-Pack) - Model: Deco M4R Ver: 2.0

The Deco M4 devices were installed in Access Point mode, 
with ethernet CAT5 backbone connected to a port on a 
TP-Link 450Mbps Wireless N Router - Model: TL-WR940N (EU) Ver: 6.0

The SSID: TP-Link A2C6 from the TL-WR940N is used throughout the house.

All devices are on the same floor level

At first every thing seemed to work fine, but then after a few hours the connection speed on each device deteriorated and then went completely off. 

All three devices have a white status LED  displayed when this happens. Al devices are affected when this happens, namely:
3x Android phones 
2x Smart TVs
1x Laptop
2x Solar wi-fi network devices.

I then perform a "Complete Reboot" with the Deco Phone App and the Wi-Fi network works fine for a while. 

Deco M4 devices have been installed now for just over a week and I have not had a single day day without Wi-Fi connectivity issues. 

All device firmware is current version.

The Deco M4 devices are wall mounted, but even putting the devices in the center of the room does not make a difference to the issue experienced.

I am at a loss of ideas to resolve this issue.

Please can you advise what I can do next to get this network working properly and consistently.