Thanks for all replies, so:
The problem was that my raspberry PI was not using DHCPD server. @deHakkelaar pointed me to the right topic. I am using Debian 12 and dhcpd didn’t work for me. That was a solution for me: Trying to understand why Pi-hole DHCP no longer works with my Virgin Hub 3.0. - #9 by deHakkelaar
This topic can be closed now