Steps to secure a DNS Cluster

I recently setup a DNS Cluster in order to get some redundancy. The issue I normally use managed servers, but since I thought these would be pretty easy to operate I did not get them managed.

My issue is, after recently setting them up...sometimes the brute force detection on the dns only servers blocks the main server that is trying to sync to them. Are there any specific things I should do with the firewall/brute force detection in order to prevent this from happening?

Also, suggestions on best steps to take in order to secure/harden a dns only server would be awesome if anyone feels so inclined . I've found a few guides on how to secure a centos server, and how to secure a cpanel server, but most of them are pretty old...and wasn't sure if they process would be any different specifically for dnsonly version of cpanel. Thanks in advance for any help!