How do I configure DNS now that OS X Server doesn't support it?
Locating configs with lsof

You could use lsof to try and find out what files the running DNS server is currently using and perhaps find out where the existing files are located.

$ lsof -p $(pgrep -f &lt;name of DNS server&gt;)


NOTE: I believe the configuration files are typically located here ,/Library/Server/, so you may want to look through there for them.

You may also want to look at the following resources regarding this from Apple:

BIND

DNS on Mojave server


  DNS is no longer included in Server.app so you will have to do it
  either on a Mac running an older version of macOS and hence older
  version of Server.app or do it via the command line.
  
  If you manually install BIND then at least the config files will be
  the same as previously used in Server.app although in a different
  location


Based on this it sounds like you may be able to simply take your existing configs, if you can find them, and use them directly with BIND.

Service Migration Guide

Also take a look at this guide put out by Apple titled: macOS Server
Service Migration Guide v1.2.

References


Setting up a local DNS server on OSX
Configuring Your Own DNS Server on Mac OS X