If your DNS system is down, then there are numerous of potential reasons.
Start off by logging into NPMPlus panel and search for the dnsapi route.
Since the DNS api is installed on the NPMPlus server, coincidentally, it should have the same local ip as the npmplus server use.
If the ip doesn't match, then the problem is already found.
The url to the dnsapi is https://dnsapi.yourdomain.tld , the port is 8443.
When you visit https://dnsapi.yourdomain.tld it should state "api is running", if it doesn't state anything and is pointed to the proper place, then it is probably not running.
Open your npmplus container and write "pm2 list" to check wether or not it is running.
If it isn't running, there's probably a reason for it, write "pm2 logs dnsapi" to read the error logs, discus them with other people if you need to figure things out.
To restart it, write pm2 restart dnsapi.
Sometimes it might help to give it a flush, do so by writing pm2 flush dnsapi and pm2 restart dnsapi