So right now Syncthing is configured to its defaults & thinks its default data dir is /root/Sync. That’s not your USB stick; that’s a dir on the onboard flash.
You need to edit the openwrt syncthing conf to point to your proper mount & its Syncthing dir. If you’ve already done that then try Syncthing GUI → Actions → Settings → General → Default Configuration [ Edit Folder Defaults ]. Input the full path to the Syncthing data dir that matches the one fr /etc/config/syncthing.conf.
See my example conf above.