https://techcrunch.com/2018/11/28/google-fi-now-supports-most-android-devices-and-iphones/

Google is making a major move to expand the availability of its Fi wireless service.

It’s been a few years since Google launched Project Fi with the promise of doing things a bit differently than the large carriers. Because it could switch between the cell networks of multiple providers to give you the best signal, the service only ever officially supported a select number of handsets. You could always trick it by activating the service on a supported phone and then moving your SIM card to another (including an iPhone), but that was never supported.