In Debian 12, but more generally in systemd >= 250 (see this Debian bug and links therein), the naming scheme for interfaces change from ethN to enXN (N = a number) for Xen interfaces. As a consequence, the script /gandi/init executed at startup by this package is failing and the server has no active interface (hence no external connectivity).
When I replace the interface names eth to enX in various scripts, it works. A non-exhaustive list is:
/gandi/init (outside of this package)
/etc/gandi/manage_iface.sh
/etc/default/gandi
/etc/udev/rules.d/86-gandi.rules