Zeiss Ikon Nettar 518/16 shutter stuck
You need to take the lens and shutter assembly off the camera/bellows. Its probably held on in the rear with a locking ring. I worked on a similar camera, the zeiss super ikonta and that was what held the assembly. Then you should remove the lens cells to access the shutter. Use a spanner again and take off the front and back cells. In the front of my super ikonta there was a front lens ring that was secured by set screws your may or may not have it. Then the simplest thing to do without going further to disassembly, spray some naphtha onto the blades and try working them. It might just loosen up any crud and get it working again. This fix is not a good way to do it, as stuff that gets loosened might migrate and end up breaking something else. But at $18 its not a big deal. If you have never any camera repair it might be better to send it out and pay someone to get it working. Never force anything, take your time, and use the right tools.