Although it’s for a different model QNAP, the solution should be the same or very similar to the one mentioned in the following earlier thread: https://forum.syncthing.net/t/qnap-tvs-h874x-i9-64g-age-old-failed-to-setup-inotify-handler-error-even-after-adding-inotify-max-user-watches-204800-to-the-sysctl-conf-file/21484
(It’s well worth carefully reading the entire thread before making any changes to your QNAP.)
Since you said things aren’t syncing, insufficient inotify settings wouldn’t prevent that from happening, it’d just mean that Syncthing would have to rely on scheduled rescans to find changes to sync. So you might still have a sync issue after reconfiguring inotify on your QNAP.