In addition to my post. I discovered that my new iPhone 11 via WiFi is connected with my Connectbox and not with my DecoM4. Maybe this helps you.

Recently I bought a iPhone 11. And on that device I now have the message "weak security" again. Only on that device! I can turn WiFi on and off as often as I want, the message keeps appearing. It...

Hello, I think my problem is solved because I don’t see the message anymore. If it happens again I will ask your help again. For now thanks and kind regards, Jovanwe

@TP-Link_Deco hello, did you receive the screenshot to support my question about the firmware ("Not available in this firmware")? kind regards, Jovanwe

One of my connections according to my Deco M4-set results with the text "Not available in the present firmware". what does this mean? kind regards, Jovanwe