Yes, I tried rebooting, no joy.
Just to clarify, when you say "In the Icons section" are you referring to the Plasma Widgets section?
No, I am referring to the Information section, and within it, the Icons heading, with four tickbox options underneath.
What I actually try to achieve is creating a new panel that is a duplicate of the default panel. The default panel has four icons in the tray: Klipper, the connection icon, Volume Control, and the notification icon.
The tray in my duplicated panel still misses Klipper and the connection icon, and I don't see a way to put them there.