VNC connection to iMac after startup not possible

iMac M1 24" 2021, macOS 12.4, fixed IP plus HW firewall

I need to connect my iMac from remote by VNC.

Everything works fine as long the user is logged in on the iMac, even if the iMac is sleeping. If the iMac is showing just the login screen (after booting, rebooting or logging out the user) no connection could be established anymore.

My previous iMac with at least High Sierra or maybe Mojave was able to VNC in the logged-out state. I am not shure if I had macOS Server on that machine. Is there any switch I can turn to re-enable VNC after booting under Monterey?

Thanks for any help.