Currently Ant Video downloader for Opera is not hosted on Opera add-ons. Starting from version 65.* Opera has disabled installation of out-of-store extensions in .nex file. You will now need to install the extension manually from .zip archive.
Download the latest version of Ant Video downloader for Opera by clicking at Install for Opera button at https://www.ant.com/products or by selecting Save Link As... on desired AVD version at https://archive.ant.com/opera/
Extract contents of .zip file into a directory where Ant Video downloader will reside
Open Menu — Extensions — Extensions (or press Ctrl+Shift+E)
Enable Developer mode (the switch is on top right)
Click on Load unpacked button
Navigate to directory with Ant Video downloader that you previously extracted, and click on Select folder button
Note: Do not delete the Ant Video downloader folder defined in step 1, otherwise the extension will not be loaded.
By default, Ant Video downloader is disabled in private mode. If you would like to use Ant Video downloader in private mode, select option Allow in incognito on Opera's Extensions page (Ctrl+Shift+E).
After you installed the browser extension, you need to download and install the Native Application.