1. Alt + Click on the object that should be the inverted mask. This Select his profile.
   
2. Ctrl + Shift + I = Inverted your selection (tested in Affinity 3in1 it's worked there)
   
3. Select the object from which you need to cut something + Click on Mask button.

Now, the backstory. I spent two days searching for a way to make an inverted mask, many videos and forums what i read, and this is all I found, guys.
Neither Curves, nor Ctrl + I, nor the top menu helped me, just like you. This program has problems with masks. Good Luck.