On some provider (https://www.nine.ch) disabling IPv6 makes unbound
not respond to the DNS queries.
Also, I was unable to test with prefer-ip6 to 'no' because unbound fails:
unbound[5657]: /etc/unbound/unbound.conf.d/cloudron-network.conf:8: error: unknown keyword 'no'
unbound[5657]: read /etc/unbound/unbound.conf failed: 3 errors in configuration file