There are several reasons why this issue can occur, and it all depends on how you configured Control D, and on what type of physical device.

Legacy DNS

If you've used Legacy DNS, there is a high chance that you are subject to the IP Authorization problem. Simply follow the instructions on the linked page. This is by far the most common issue that can occur.

DNS Hijacking

Some ISPs and corporate or school networks hijack Legacy DNS and redirect it to their own resolvers. If the client happens to be on such a network AND uses Legacy DNS, there is nothing you can do about this except switch to a Secure DNS protocol like DNS-over-HTTPS or DNS-over-TLS.

Secure DNS

If you're using Secure DNS protocols, there should be very few reasons for it not working, except for routing issues. Try pinging dns.controld.com and see if you get a response, if you do not, that suggests the IP is not routable from your network. This should be rare, but if it occurs, send us a traceroute.

If the IP is routable, and it still doesn't work, then you may have typo-ed the DNS resolver, or deleted the associated Device from the web control panel. This will break all DNS resolution, except for controld.com. If this works, and nothing else does, then this is definitely the problem. Simply re-create your Device and update your resolver.