You don’t necessarily need a new custom group, but you do need a new template. You create a new settings template or duplicate from an existing template, then configure that template with the settings you want. Now modify the device settings to use that template, or you can if you prefer move that device to belong to a group that is configured to use that template. The PC will pick up the new settings in about 10 minutes or so.
I like having a “Testing” template to mess around with settings before I apply to my other templates.
Unlike other policy-based consoles, Avast Business Cloud doesn’t have hierarchy inheritance to customize one setting for a subset of machines. So a separate template is the way to go for applying options differently to different PCs.