The article you were reading when you started, which is linked to your post on the forums, describes the exact item you need to connect a genuine Thunderbolt-2 display like the Apple Thunderbolt Display to a Mac with ThunderBolt-3 or -4 ports.
Make certain you do indeed have the Thunderbolt Display -- the extremely similar Apple Cinema Display can be differentiated by its lead in cord including a USB-A input. The Apple Cinema display is a Mini DisplayPort display, and DOES NOT WORK with the ThunderBolt-3 <-> ThunderBolt-2 adapter.