I have a sizable deco network, most on my devices are capable of WPA3. However I have some M9 plus still in my network. 

I found out that because of this reason I cannot select WPA3 for my wifi network.
It would be nice if I could limit the use of the M9 to the IoT (and/or guest) network which then would be limited to WPA2. So I could use WPA3 on my main wifi network.

In my specific case I have wifi kWh-meters in the fusebox and an M9 in the closet with the fusebox, to improve the connection. So limiting it to an IOT network seems practical.

Sidenote: it took some searching to find this out. It would be preferable if the WPA3 option was visible but greyed out (not selectable) together with an (?) next to the option to provide the explanation why this option cannot be selected.