Couldn't Connect Private DNS (Own DNS)

I have created my own DNS resolver for blocking some web pages and I can't figure out how to use it on Android. If I configure the DNS of any other device to use my DNS, it works like a charm (I've tested this both Windows and Ubuntu). I tried configuring it in the Private DNS section of an Android, but it keeps saying "Couldn't Connect". If I use 1dot1dot1dot1.cloudflare-dns.com instead, it works. So, my question is: What are the requirements for a DNS Provider to be configurable in the Android's Private DNS section?