Please does anyone have any experiences or recommendation for a personal GPS tracking solution?

My father-in-law has dementia which is slowly getting worse and he is starting to have a tendency to wander and not remember where he lives. I'm looking for some sort of way that the mother-in-law can keep track of where he is. There seems to be quite a range of options out there with widely varying prices and technical complexity but nothing jumps out that immediately ticks all the boxes.

My Wishlist/thoughts:
- must be available and work in New Zealand
- the portable device to fit on a keyring or in a wallet
- simple to use - they have little aptitude or desire for learning technology
- they don't have an internet connection or PC at home (although I won't rule out their having to get a smart-phone with a data plan)
      *alternatively is there a 3rd party monitoring service that MIL could phone to locate FIL when he's gone walkabout
      *us kids could act as this 3rd party but this is less desirable as we all work full-time and may not be available when the need arises.
- zero or minimal monthly subscription charges (their only income is the government pension)

Yes, we do intend to get in contact with the dementia support organisations to see what they have to offer and say, but I'm keen to hear what works for others here.