You simply have to add with vim /etc/config/network
at the end.
config interface 'lan'
...
option gateway '192.168.178.1'
list dns '192.168.178.1'
The question is, why I never had to do this (or remember this). Maybe a slightly different way to change from openwrt-network to Fritzbox network. No idea. Maybe an issue with the linux-pc-configuration and static ip and dhcp.