There’s no pop-up. It’s on by default, and you can check your settings in Preferences > Backups [another thing that’s explain on the backup page linked above]. There have been developments in the backup functionality, especially 3 years ago in 2.1.50 – but the feature goes back at least to the release of Anki 2.1.0 in 2018, so I think you’ve got it.
Are you looking in the right place – Managing Files - Anki Manual ? Have you gone in through the Anki menus to get to the Backup interface – Files/Anki > Switch Profile > Open Backup.
How long ago was “used to sync”? Have you gone to AnkiWeb and checked to see if they are there – https://ankiweb.net/decks ? What’s the name of the deck, and what’s some text that’s in a note?
Sometimes it’s helpful to start over from the beginning – what fiddling and moving were you doing?