Please do not open port 53. Without proper counter-measures, open resolvers contribute to DNS Amplification attacks. If you have an open resolver, I guarantee that it is being used maliciously. Please close your port 53 and use a VPN to securely access your pihole.
> A Domain Name Server (DNS) amplification attack is a popular form of distributed denial of service (DDoS) that relies on the use of publically accessible open DNS servers to overwhelm a victim system with DNS response traffic.
DNS queries are much smaller then DNS responses. Making a huge amount of queries uses less bandwidth then uses to respond - making it a prime candidate for DDOS attacks. Look at your logs, no doubt you will see a large number of requests for various hosts. This is your system being used to attack people. Please close the port.
DNS Amplification Attacks: https://www.us-cert.gov/ncas/alerts/TA13-088A