The tensioner needs to come appart. I hope there's a tutorial for yours availavle and assembly lineup for the parts. Singer tensioners are usually very similar, but they are not all exactly the same. Some have a setting screw (grub screw?) on the outer nut part, others not.
You can do a bit of investigating behind the faceplate. When you lift the presser foot lever it should push on a tiny metal pin, the pin pushes the disks appart.
When you do this remember there is tension when the presser foot lever is down, and no tension when in up position. It's very basic, and I know you know it, but so easy to forget when we are trouble shooting.