WiFi connects and disconnects multiple times when enabling WiFi

When I have the issue, then my Fairphone connects and disconnect and so on…
In parallel I can connect to the guest WiFi across the same router. But then of course I do not have access to other resources in my privat network.
Some time I can work around while change the WiFi channel to a dedicated number but not too auto. Auto will then not work.
But today I checked all channels manual, with no result.
I disabled 2.4 and try with 5 Mhz only, but this does not help to. This is the same with 2.4 Mhz to.

So I am out of ideas!