What happens in a Mesh when I mix Wifi 6 devices with devices which do not support Wifi 6?
Do I loose the features of Wifi 6 / AX then?
I will replace my old routers with new ones, which support AX like Asus TUF AX6000 and RT-AX53U, but a ZyXEL NBG6817 or a Netgear Nighthawk X4S R7800, I have both, I would like to use too.
What can I do, that I do not loose Wifi6 features in a Mesh?
You won't, but only ax devices will profit from ax features. Each device will establish the fastest connection both devices involved can support.
Probably I don't understand good enough how Mesh works.
Let's assume the devices are all in a line.
Main router with AX ---- Extender without AX (2m distance) ---- AX-router (10m distance)
Does the "10m-router" without AX use the signal of the "2m extender" without AX?
In the meantime I found Recommendation WDS 2x Routers Wpa3 Wifi6 min. 4x Gbit ports - #31 by linuxuser So I bought 2 ZyXEL WSM20 and then there will be all AX-devices.
In a 'mesh' (in the sense of 802.11s), you won't really influence which route the distant route takes, it may do whatever it prefers (or both at once).
In a WDS/ 4addr setup, you can force it to use one BSSID (macaddr), so if the distant router doesn't need the extender to reach the internet, you can make it use the main router instead (if that makes sense or not depends on your environment).
If the traffic has to pass the extender, that will be the bottleneck - clients may still connect to the distant router via 802.11ax, but the effective speed is going to be determined by the slowest link in the chain (including the repeater effect slowdown).
In WiFi land, communication is always between client and AP, meaning two wifi clients will always talk through the AP to each other (not directly). Each of those links establishes the connection on their own terms, meaning an ax client will happily use ax to connect to its ax AP, while a g client will only manage to connect at g speeds/ 54 MBit/s to the same AP at the same time. If both clients want to talk to each other, they will each do so at the maximum speed supported by the individual client and the AP - just that the effective throughput will naturally be limited by the weakest link in the chain.
So this means, try to avoid non-ax devices. Not sure at the moment, what will be the configuration after thinking about the whole situation, but eg 5 ax routers and 1 none-ax means I have a bottle neck and it is better to use a cheap ax router / ap like the ZyXEL WSM20 instead of a ZyXEL NBG6817 ?