Ubuntu AP not assigning IP address

I am running Ubuntu 20.4 and via the settings GUI I configured it as access point to share the wired connection to other devices (ubuntu computer, android smartphones).

The access point starts, the other devices see the SSID, password is correct, but the access point does not assign them an IP address; client devices wait for an IP address forever...

With

ifconfig -a 

I see the AP ip address, netmask and gateway (10.42.0.1/24)

I statically assigned addresses to the other devices (10.42.0.2...5) and they can access the internet.

What shall I do so that the AP serves ip addresses to the connecting clients ?

One would expect that all needed pieces work automatically... Shall I install a DHCP server ?

Any help highly appreciated.

P.S. I have a second, less relevant question :

nm-connection-editor

doesn't allow me to change channel number. The field is not editable.

What shall I do to chanhe channel number ?

Again, thanks for your help!