This now looks different than in for instance 9.21 where you had the “bulk configuration” - “multi-edit” menu under “AP”.
In version v10.xx you can go to “AP”s, then select the APs you want to edit with a checkmark and below in the “Actions” button, select “multi-edit”.
Here first the correct country needs to be selected.
Before being able to select “custom” under the channel plan, you will also have to configure the “Radio Mode”, so for instance “a/n/ac”.
Once the radio mode has been selected, you can select “custom” under “channel plan” and the button “configure” appears to select the specific channels.