@David-TP 

What happens is, that when network quality is in bad state is that internet connection speed drops from 250 MBit to 30-60 MBit with high latency > 100ms instead of < 20ms. Then I can't stream video chat like Zoom, Teams, OpenTok without video stuttering or hanging. I have to disable video. Audio is mostly continuing to work fine. Also some or all ressources/assets of websites will not load (external css and script files), mostly for foreign webpages (where server location is not located in or near Frankfurt/Main, Germany). 

After "optimise network" all is woking again. Sometime for some days, sometimes only for hours. Sometime network quality gets bad within minutes, sometime it is perfect for some days.

I had seen up to 7 other wifi networks on the same channel while in bad state. Even in perfect state there are at least 4 other networks. I am using 2,4 and 5 GHz with WiFi 5 preferably if possible. As I said I have ethernet backhaul between two decos in AP mode with FritzBox 7590 as (non-wireless) router. There are around 20 devices online at day. Only 2 connected to 1st deco (nearest to FritzBox) on 2nd floor, and the other connected to 2nd deco on 1st floor. For immobile devices I turned of Mesh in deco app because otherwise they will connect to the furthest away deco with low bandwith then (underfloor heating massively attenuates signal), even if the faster deco is within 5 meter of range.

Using the both decos without ethernet backhaul was not practical because speed and even whole connection dropped frequently to <  5MBit with 5 GHz backhaul. Even if the backhaul was serving around 400 MBit while working.

As long as I used FritzBox with FritzRepeaters, I noticed a lot of "TDWR" warning in channel selection. Radar was a frequent problem with Fritz devices.