I faced the same issue with new functionality to slave a point from TGP. As I discovered, cage/uncage has nothing to do with slave/unslave to WPT. Here is how slave to WPT with TGP is working:
You need to press WPDSG to slave a waypoint, for example WPT1.
When you want to slave an another waypoint (WPT2), you need to press NWS ( nose wheel steering button). And after that, press WPDSG again. Now it will slave to WPT 2. That's it. You dont need to cage/uncage. Good luck.
And one more very important thing: Your should make your waypoints altitude to 0 (zero), or your TGP will be slaving a point somewhere in the air.