Could you figure a way to slip one of those newfangled Apple Airtags into a collar or harness pocket?
Those have to be near an iphone to report their location. The Fi collars connect to cellular network themselves so they work anywhere with a signal. They kind of hide it on the website until you add to your cart but it costs $99/yr for the cell service, a bit less if you do more than a year. They only update location every 2-5 minutes unless you put it in “lost dog” mode and then it will update every minute. I know someone who has one and likes it for their purposes. Just keep in mind the refresh rate and that it won’t work where there is no cell service.