Hi I am using deco m5 for few years, have 3 deco pieces, all connected through cable, still, they have perfect signal over wifi, too. Till few weeks ago, everything worked fine, but then, the main network started doing problems (I did not do any modifications, maybe did deco update).

Basically, the devices on the main network are loosing connection while I did not experience this on guest network which works completely fine. In practise, if I connect with phone or ipad, opens browser or ie youtube, it looks like device is not connecting to internet, although wifi connection on device is full In the deco app, devices are online in the list, but with no issue, expect that I just see device is not sending/receiving anything from internet. 
 

Do you have any suggestion? I removed devices and added them again, I forgot wifi on the devices, I setup the network from scratch, nothing helped. I have around 20-25 devices connected to main network, 2-5 to guest network.

Mode: Wifi router

Wifi settings: WPA2, band 2.4 and 5 ghz