The following instructions are for setting up DoH with Opera on the desktop. The steps are similar for setting it up on Opera mobile, but if you are using Android and have blockerDNS set up as your Private DNS provider, there's no need to also add it to your browser.
Go to Go to full browser settings
Search for DNS using the box at the top-right
Enable the Use DNS-over-HTTPS instead of system's DNS settings option
Change the provider to custom and then put in your custom URL: