Adding
iface eth0 inet6 dhcp
in
/etc/network/interfaces
does the work.
So I guess by adding a line
iface eth$ETH_DEV_INDEX inet6 $eth_dhcp_static_text
in position 2396 (and in 2404 for wifi “iface wlan$WIFI_DEV_INDEX inet $wifi_dhcp_static_text” ) in
/boot/dietpi/dietpi-config
under Network_ApplyChanges() function will make the change permanent for the future.