With DirectAdmin you can utilize the built-in Multi-Server function to sync DNS records to additional servers.

You can install this free plugin DirectSlave, to add additional edge DNS servers without additional DirectAdmin licenses. 

Multiple DirectAdmin Server with no clustering

To use this setup, simply input N# of provisioning server (Your DirectAdmin server) under your WHMCS. For the Nameservers setup, you would use the nameserver hostnames prefixes that are pointing to each of your Directadmin servers.

For example if you have three DirectAdmin servers; da1, da2, da3. and you have three sets of nameserver prefixes; ns1/ns2, ns3/ns4, ns5/ns6.

ns1/ns2 would point to da1, ns3/4 would point to da2, ns5/ns6 would point to da3.

Simply insert the respective nameserver prefixes for each of your DirectAdmin server.

Single DirectAdmin Server with multiple Edge Servers

To use this setup, simply input one provisioning server (The main DirectAdmin server) under your WHMCS. For the Nameservers setup, you would use the hostnames that are pointing to your Edge servers, and not your DirectAdmin server. 

For example if you have four edge servers, then use each one of the hostname to point to these 4 edge servers. ns1/ns2/ns3/ns4.yourdomain.com, each pointing to different Edge server's IP address.

Single DirectAdmin Server with multiple Edge Servers with round-robin

To use this setup, simply input one provisioning server (The main DirectAdmin server) under your WHMCS. For the Nameservers setup, you would use the hostnames that are pointing to your Edge servers, and not your DirectAdmin server.

For simplicity sake, you have four edge servers; Edge1, Edge2, Edge3, Edge4. and two nameserver prefixes; ns1/ns2.yourdomain.com.

For ns1, you would setup round-robin DNS entry to Edge1 and Edge 2. And for ns2, you would setup round-robin DNS entry to Edge 3, Edge 4.

With this setup, you would only use enter ns1 and ns2 in the Nameservers setup.