There are various solutions to access things via WebDAV, one being rclone, another davfs2.
I wouldn’t bother with cifs if webdav is an option.
- [Guide] Mount Nextcloud (WebDAV) folders locally with davfs
- [Guide] Setup & Mount WebDAV-Resource with Rclone on SailfishOS
EDIT: Sorry, I assumed FritzNAS supported WebDAV as a server.
It appears it only supports it as a client (to some online provider), and service on the LAN is Samba/CIFS only.