iMac 2019 internet sharing via WIFI - setting of Wi-FI channel not working

when setting internet sharing via WIFI, in the options to set the Wi-FI the pop-up for channel selection is not working and therefore is not possible to select the channel for the Wi-FI connection to be created.

Because of the lack of possibility of selecting the channel, the HW goes by itself as default to 2,4GHz Channel 11, 20MhZ.

Setting a 5GHz channel is not posible and hence the internet sharing remains locked to the slow 2,4GHz connection.

how can this be solved?

equipment and iOS

iMac 2019 Retina 5K, 27-inch, 2019

iOS Big Sur 11.6