Hey guys!
My system tray icons are not showing up on my second monitor, at least not all of them.
On my main monitor all the icons are showing up.
Windows 11 (24H2, 26100.4770)
Thankful for any help!

Many icons are missing, but for me most importantly Cpu Core Temp app, that shows what temp the cpu is at.

I got the same issue here and I will to try to clarify.
The tray icons from Fan Control (temperature readouts) are not shown on DisplayFusion multi‑monitor taskbars unless those icons are hidden in the Windows tray overflow on the primary taskbar. When I set them to “always show” on the Windows primary taskbar, they disappear from the DisplayFusion taskbars on the other monitors.

Screenshot

1. Primary taskbar (Windows): “Always show” enabled for Fan Control icons
The Fan Control temperature icons are pinned to the primary taskbar (not in the overflow).
Result: They do not appear on the DisplayFusion taskbars on the other monitors (see 2.)

2. Secondary monitor (DisplayFusion taskbar) while icons are “Always show” on primary
The DisplayFusion taskbar’s system tray is enabled, but the Fan Control temperature icons are missing.

3. Secondary monitor (DisplayFusion taskbar) after moving icons to the primary taskbar’s overflow
I set the Fan Control entries to the overflow on the primary taskbar (not always visible).
Result: The Fan Control temperature icons now appear correctly on the DisplayFusion taskbars.

Using DisplaFuison Pro 11.1.1 on Windows 11 (24H2) [26100.4652], FanControl v228

I cant see the provided screenshot as its protected but i'd like to know where you are changing the icon settings from always show to never show etc etc, i dont have such thing?

That's a Windows 11 taskbar setting. I have the German version, but in English you might label them like this:

Additional taskbar icons
(for the icons you choose to show permanently on the taskbar)

Hidden (overflow) icons
(for the icons tucked away in the overflow menu)