The default configuration acts as a caching server.
Simply uncomment and edit /etc/bind/named.conf.options to set the IP addresses of the DNS servers you want to use; such as your ISP’s DNS servers:
forwarders { 1.2.3.4; 5.6.7.8; };
NOTE: Replace 1.2.3.4 and 5.6.7.8 with the IP Addresses of actual nameservers.
To enable the new configuration, restart the DNS server:
sudo systemctl restart bind9.service
See dig for information on testing a caching DNS server.