1
Open the Settings App, go to Apps - Installed Apps.
Check in there if there is an app like Bing Desktop Wallpaper, if so, uninstall that app.2
2
Then, in the Settings App, go to Personalization - Themes.
Ensure that is set to the default Windows theme.
3
Then you can set your preferred desktop background wallpaper in Settings - Personalization - Background, and it should not change dynamically.
4
If you want to set one of the default wallpapers ad your Desktop background, open File Explorer.
Paste this into the Address bar and press Enter.
%SystemRoot%\Web\Wallpaper
In the resulting folder, that is where all the default wallpapers are stored.